Thumping when running engine on stands

[Graham]

While trying to diagnose my clunk into reverse, I have had the rear of the car (85 300D) up on jack stands. In trying to locate source of clunk, I ran engine and for short period had a loud thumping from , I think, passenger side front.

At moment, I am checking vacuum systems and started engine, just in Park. Again I had a thumping noise from passenger side front which gradually subsided after a few minutes.

There is likely more weight on front, but still haven't figured out what is thumping. The motor mounts were replaced 5000 miles ago and still look like new. At that time, the engine dampers and their mounts were also changed. All either OE or equivalent. Transmission mount just replaced (not OE )

Anyone had similar thumping when rear jacked up?

[resago2000]

check the engine mount bolt. it may have backed out.

[Graham]

Originally Posted by resago2000

check the engine mount bolt. it may have backed out.

Good point. Will check all the bolts once new axles are in and I can get under front of the car



PS: Wrong car, but same idea

[Graham]

I checked the bolt on side the noise was coming from, and it was tight. Couldn't get at other side.

While about it, I did a few tests:
- First I put a floor jack and 1x6 plank under oil pan barely making contact. Started car and it made the clunking noise. Gradually lifted pan a bit at a time. The noise diminished and then went away. Took about an inch of lift.
- Lined up top of valve cover with top of fender (by eye) and lifted the pan. It moved up a bit (hard to measure) before the fender started to lift.

Not sure if this is normal. The car body is supported on jackstands at rear., front wheels on ground. Rear tires would be about 9" off ground. So there is quite a bit of load on the front end springs. I guess that would bring the engine down closer to front suspension?

Maybe what I am seeing is not unusual? Hard to inspect front end with car supported as it is, but I have checked under car as best I can, and can't see any place that shows signs of contact. Will check again once I get new rear axles on.

[Graham]

With car back on wheels, this thumping has more or less gone away. I think it is caused by the exhaust being very close to the frame rail or whatever that is that it almost touches. When car first starts, engine is a bit rough but as it settles down, the noise goes away.