I have 76 300D that makes a thumping noise that I was certain was the exhaust pipe hitting the chassis/frame. I know this noise and was very surprised to find the exhaust pipe was tight and not the source of the noise. It is related to engine shake at idle. The car starts/runs great but has always had a rough idle. It only makes the noise when the car is stopped in drive. The car will stop making the noise if I shift to neutral. I think there is more to it than the change in rpm caused by putting it in neutral. The car is very solid (no rust, never wrecked) and I have simulated the conditions with the car in the air, but I cant find the source of the noise or anything loose that is striking the body. Has anybody else had this annoying noise? From under the car it appears to be coming from on top of the transmission, but I cant find anything loose up there. From in the car it sounds and feels like something is striking the car under the drivers seat. I have tolerated this noise for about 5 years and 30,000 miles thinking someday, something would come loose or break and I would find the noise. Any thoughts would be appreciated.
John