I have a 99 C230 kompressor and I can hear this clunking noise when the car goes to a stop or take the foot off the gas and then step on gas a little bit right after. The dealer replaced the modulator, but it is still doing this intermittently. before they replaced it, it clunked when the car went down hillls. The car has 27k miles and what is wrong with the transmission? can it be front flex disc??

The tranny is out of my league, but does it have a lock-up torque convertor? Some of those got very noisy. Otherwise: Can you reproduce the noise by holding the brakes locked and going from forward to reverse? If so, they should be able to do the same thing on a hoist and hear the exact location of the noise, such as excessive differential play, flex joints, broken mount, etc.