Exhaust rattle/resonance when cold

[C-Mack]

I’m experiencing a very odd sounding exhaust resonance/rattling in the morning before the vehicle is fully warmed up. It’s coming from near the right rear of the vehicle and manifests itself only when driving slowly about 5 mph and you can hear it in the exhaust. I’ve checked all underside heat shields along the length of the exhaust for any looseness or panels touching (especially the large heat shield directly above the rear silencer) and except for a few tweaks everything is nice and tight and not interfering with anything else.

This problem only lasts for a few seconds and quickly goes away as the exhaust system warms up. Could it be something internal to either the center exhaust resonator or rear silencer itself? I had another person walk besides the vehicle in the morning and they described it almost as an exhaust rumbling noise as if something internal was loose and quickly expands with the exhaust heat and then tightens up? After a few seconds of this noise in the morning it never repeats the entire day until the next more and I go through the same routine and warm up and it does it again. Any thoughts on what this could be?

 

[C-Mack]

Quick update on this noise. It is indeed coming from the exhaust silencer in the rear. (Verifed by walking beside the vehicle when coasting slowly down a hill.) you can clearly hear the noise coming out of the rear exhaust tips. I can simulate the noise in park by blipping the throttle during engine warm up and as the rpms drop back to idle you can hear the popping and rattling from the exhaust.

The noise only lasts until I have to accelerate and the engine is producing more gas flow and heat at which point the noise goes away. Sort of sounds like marbles in an empty coffee can or like at highly tuned sports car that pops and bangs a bit just off throttle.

My assumption is the motor must be running a bit rich during morning cold start cycle and this is extra fuel burning off on closed throttle until the catalyst is up to full temp but not knowing the fuel mappings or how the vehicle is programmed to handle the transition from closed to open loop running I’d just be guessing. Point being, once fully warmed up off throttle coasting at slower speeds the exhaust popping doesn’t occur again until the next morning warm up cycle.

 

[David Czarnecki]

We have had the exhaust off multiple times in developing our exhaust system with Kooks Headers and Exhaust and one thing you can do is look at the heat shields around the exhaust and make sure they are not contacting the exhaust anywhere. Its simple to look underneath on a creeper for example. The heat shields are aluminum and can be bent very easily away from the exhaust system. I look under regularly after off roading and had to move them twice, not sure if they got hung up on something or not. This would be a rattle sound but not the marble situation you described in the second part of the post.

 

[C-Mack]

We have had the exhaust off multiple times in developing our exhaust system with Kooks Headers and Exhaust and one thing you can do is look at the heat shields around the exhaust and make sure they are not contacting the exhaust anywhere. Its simple to look underneath on a creeper for example. The heat shields are aluminum and can be bent very easily away from the exhaust system. I look under regularly after off roading and had to move them twice, not sure if they got hung up on something or not. This would be a rattle sound but not the marble situation you described in the second part of the post.

Thanks David, I’ve checked all the heat shields front to back and there were a couple spots that needed tweaking included the shield above the rear silencer. There is a couple tabs either side that get really close to the exhaust tips as they curve down and head to the rear. Nothing is touching or vibrating heat shield wise. This is a pop/crackle noise internal to rear silencer. I’ll try and get a video clip to show the sound better as I can replicate it with the vehicle stationary.

I’ve seen your Kook exhaust videos on YT and in some shots I can hear a similar noise on the overrun on your system but on this one it’s a lot louder and more of a marble in a can noise. I can replicate it on throttle blips when cold but once the system heats up it doesn’t seem to make the same noises.

 

[C-Mack]

We have had the exhaust off multiple times in developing our exhaust system with Kooks Headers and Exhaust and one thing you can do is look at the heat shields around the exhaust and make sure they are not contacting the exhaust anywhere. It’s simple to look underneath on a creeper for example. The heat shields are aluminum and can be bent very easily away from the exhaust system. I look under regularly after off roading and had to move them twice, not sure if they got hung up on something or not. This would be a rattle sound but not the marble situation you described in the second part of the post.

David, if you guys ever develop a bolt-in replacement for the rear silencer section “only” that produces a quiet/deeper, mellow exhaust sound free of pops and bangs I may be in the market. I don’t want the Grenadier to sound like a sports car/street racer it just seems out of character for what it is.

 

[C-Mack]

Here is a quick clip of the cold exhaust noises I'm hearing. This is stationary blipping the throttle but these are the same pops/bangs/crackling sounds I hear while coasting slowly down my drive way but noise is constant with no throttle input in the morning for about the first 50 yards or so. Once more heat is in the exhaust these noises don't seem to return when out and around town slowing down for lights or intersections. Literally sounds like marbles in a coffee can at times.

 

[David Czarnecki]

David, if you guys ever develop a bolt-in replacement for the rear silencer section “only” that produces a quiet/deeper, mellow exhaust sound free of pops and bangs I may be in the market. I don’t want the Grenadier to sound like a sports car/street racer it just seems out of character for what it is.

I can get you an axle back version, that would be exactly what you are describing. You would just have to cut the existing system and our picks up the on the original. Give me a call and I can discuss but it would essentially be half the kit. 704-682-9323.

 

[C-Mack]

I can get you an axle back version, that would be exactly what you are describing. You would just have to cut the existing system and our picks up the on the original. Give me a call and I can discuss but it would essentially be half the kit. 704-682-9323.

Much appreciated David, let me see if my stock system quiets down first after I get a few more miles on it. Haven’t even broke 300 miles on it yet. I have listened to the video clips of the Kook system (which looks awesome BTW) you’ve been working on on YT but it’s still seems a little bit noisy for my taste especially on deceleration with the popping and gurgling sounds.

That said, I ran across a sound clip of the Milltek Grenadier system and it sounds awful. It’s as if the exhaust system was sized too big for the exhaust flow characteristics of the engine giving it a hollow sewer pipe sound, your system sounds much better in that regard. Deep, quiet and mellow would be the characteristics and sound quality I’d be most interested in.

If you are still experimenting and end up other differently tuned systems please post them online and I’ll take a listen as I agree, the stock system isn’t the best sounding.

 

[David Czarnecki]

Much appreciated David, let me see if my stock system quiets down first after I get a few more miles on it. Haven’t even broke 300 miles on it yet. I have listened to the video clips of the Kook system (which looks awesome BTW) you’ve been working on on YT but it’s still seems a little bit noisy for my taste especially on deceleration with the popping and gurgling sounds.

That said, I ran across a sound clip of the Milltek Grenadier system and it sounds awful. It’s as if the exhaust system was sized too big for the exhaust flow characteristics of the engine giving it a hollow sewer pipe sound, your system sounds much better in that regard. Deep, quiet and mellow would be the characteristics and sound quality I’d be most interested in.

If you are still experimenting and end up other differently tuned systems please post them online and I’ll take a listen as I agree, the stock system isn’t the best sounding.

The axle back leaves the stock resonator and does not burp or gurgle at all. I had figured people would want that so we removed the resonator. Now I am thinking we could have offered both options but sometimes you just confuse people so not sure. It's certianly quieter that's for sure but still sounds better than stock. Enjoy the miles stacking up. I have only 3k in 6 months. Not enough.

 

[C-Mack]

The axle back leaves the stock resonator and does not burp or gurgle at all. I had figured people would want that so we removed the resonator. Now I am thinking we could have offered both options but sometimes you just confuse people so not sure. It's certianly quieter that's for sure but still sounds better than stock. Enjoy the miles stacking up. I have only 3k in 6 months. Not enough.

Good to know the axle back system is quieter. If you end up doing another video demonstrating the difference between the resonated and non-resonated versions that would be ideal. I think you’ll have buyers in both camps… those wanting a bit more racy sounding exhaust and others wanting an even more refined deeper sound over the stock one. Curious if your systems changes anything with regard to back-pressure? I would assume back-pressure would need to remain somewhat close to stock not to impact downstream readings for emissions and fuel management?

 

[David Czarnecki]

Good to know the axle back system is quieter. If you end up doing another video demonstrating the difference between the resonated and non-resonated versions that would be ideal. I think you’ll have buyers in both camps… those wanting a bit more racy sounding exhaust and others wanting an even more refined deeper sound over the stock one. Curious if your systems changes anything with regard to back-pressure? I would assume back-pressure would need to remain somewhat close to stock not to impact downstream readings for emissions and fuel management?

We designed the system to have as little to no back pressure. There is no resonator and the rear muffler is reduced by about 40% in volume. Have you figured your issue out yet?

 

[Jiman01]

My truck has always done the decel gurgling when cold. My wife had an X4 M40i that also popped and gurgled back in the day, so I assumed it was a B58 thing

 

[C-Mack]

We designed the system to have as little to no back pressure. There is no resonator and the rear muffler is reduced by about 40% in volume. Have you figured your issue out yet?

Not sure what the specific cause is but it still gurgles, pop/rattles a bit during deceleration or coasting to a stop with your foot off the throttle within the first few hundred yards of driving from a cold start. It’s not crazy loud or anything just an odd combination of sounds you wouldn’t expect a vehicle to make unless it was a sports car with a racy exhaust system.

Perhaps it has something to do with the engine being in “closed loop” rather than “open loop” when fully warmed up and is part of an emissions strategy (extra fuel) being introduced on the overrun to heat up the cats quicker after a cold start? Or maybe it’s some combination of exhaust system temperature plus condensation in the system causing the odd sounds/pressure pulses but once fully warmed up it sounds normal.

I’ve fully inspected the full length of the system and there are no loose components inside either the center resonator or rear muffler. It’s not a heat shield or anything like that the sound is clearly coming from inside the exhaust systems and I’ve walked behind the vehicle while my wife coasts it down the driveway and you can clearly hear the sounds coming from both exhaust tips.

Engine and exhaust temperature seem to be the two biggest variables and when both are warm you never hear the noise. Park somewhere on a cooler day for about an hour you can hear the noise again once you start off but it’s much quieter so I assume the system still has some residual heat in it and takes less time to warm back up. It’s more interesting than it is a problem at this stage so just living with it and the vehicle drives and performs perfectly normal.

 

[CountyV8]

I had a small rattle/resonance on start idle it was the piece of heat shield at the rear of the transfer box smallest of tweaks and it disappeared .

 

[David Czarnecki]

Not sure what the specific cause is but it still gurgles, pop/rattles a bit during deceleration or coasting to a stop with your foot off the throttle within the first few hundred yards of driving from a cold start. It’s not crazy loud or anything just an odd combination of sounds you wouldn’t expect a vehicle to make unless it was a sports car with a racy exhaust system.

Perhaps it has something to do with the engine being in “closed loop” rather than “open loop” when fully warmed up and is part of an emissions strategy (extra fuel) being introduced on the overrun to heat up the cats quicker after a cold start? Or maybe it’s some combination of exhaust system temperature plus condensation in the system causing the odd sounds/pressure pulses but once fully warmed up it sounds normal.

I’ve fully inspected the full length of the system and there are no loose components inside either the center resonator or rear muffler. It’s not a heat shield or anything like that the sound is clearly coming from inside the exhaust systems and I’ve walked behind the vehicle while my wife coasts it down the driveway and you can clearly hear the sounds coming from both exhaust tips.

Engine and exhaust temperature seem to be the two biggest variables and when both are warm you never hear the noise. Park somewhere on a cooler day for about an hour you can hear the noise again once you start off but it’s much quieter so I assume the system still has some residual heat in it and takes less time to warm back up. It’s more interesting than it is a problem at this stage so just living with it and the vehicle drives and performs perfectly normal.

One thing I did notice is that once I put on a tuner, both the TDI and the Agile one, any extra gutgle went away as well. Just a little after thought. I actually like that sound coming off an exit ramp at high speed and deceleration.