Transmission shift quality can be attributed to the adaptive shift learning constantly changing shift points and engagement pressure ramp time depending on driving styles, changes in load and driving environment. Some modern electronic transmissions may take longer to change shift quality if there are multiple drivers with significantly differing driving styles. Modern transmissions like the 8HP ZF rely on solenoids to control shift quality while older non electronic and very early electro-hydraulic transmissions tended to have relatively fixed shift points and shift quality relied on hydraulic valving, springs and various sized orifices within the transmissions valve body.
For some modern transmissions an owner can improve the shift quality by going to a flat straight road and from a standing start lightly accelerate until the transmission works through all the gears then let the car slow down through the gears without the brakes. This may have to be done several times to allow the transmission to readapt. The owner can experiment with this and do the opposite, hard accelerate and slow quickly through the gears several times, add a trailer and see how the transmission quality changes when the car is empty again until the transmission re-adapts. Any further mechanical clunking will have to be investigated as it could be loose components in the drive line or steering related.
