I have a 1991 940 turbo when im at a dead stop and start going something rumbles underneath car. have no clue what it could be.

 

more info, could be anything from a heat shield to a motor mount. More info

 

If it's a tinny rattle it could be the cat, if it's a rocks in a suitcase rumble it may be the center bearing and/or its donut. Those sometimes can be shimmed, there was a TSB about it.

 

Does it do it under heavy load in the car? Or right around Black/White Boost on the gauge? Mine does that only when i have more than 3 ppl in the car, and is right around the mid boost on the gauge, i knwo what your saying, it feels like the turbo is struggling to push out, and kind of rattles the whole car. Wish i coudl tell you what it was...

 

it feels like i start driving on rumble stripes for a second.

 

vacuum leaks, slippin tranny? ur not givin much to go on

 

Based on a similar problem I had recently, I'm going with driveshaft angle. As JerryC101 mentioned, there is a service bulletin regarding this problem ( I found it on ALLDATA). My '91 745T had developed a nasty highway vibration, which I found to be the center bearing rather than U joints, as I first suspected. The book said that the bearing needs to be pressed out with a hydraulic press - a shop job. Since I couldn't afford to take the car off the road -even for a day- at the time, I just grabbed a complete 2 piece driveshaft from the wreck yard for $35. This cured my highway vibration, but I now had a hard shudder whenever I started from a dead stop unless I just featherfooted it - more throttle, more shudder. I remember reading something about this in an old IPD catalog. It is supposed to be a common problem in cars that have been lowered, cars that are heavily loaded, or cars with sagging suspension due to age. They suggested "shimming" as a remedy and also said that in worst case the driveshaft would have to be shortened. It wasn't clear exactly what had to be shimmed, but in the course of further research I stumbled across the service bulletin.

Basically, there are four bolts supporting the center bearing/carrier assembly. Per the bulletin, I obtained longer bolts (about 1/2") of the same thread pitch (got mine from a fastener shop that had every nut and bolt known to man) and 4-5 flat washers per bolt. I removed the 4 bolts holding the center bearing assembly and replaced them with the longer ones, with 4 washers between the car body and the bearing assembly, thus lowering it and changing the angle of the driveshaft in the middle to more of a "V" shape rather than a straight line. I apologize if this is not a good description; I don't have a digital camera yet. However, if you actually get under the car and look, this will probably make more sense. Anyhow, depending on how you place the washers you can bring the driveshaft either up or down in the center. The bulletin said you may have to play around with the number of washers to get it right, although I initially used 4 and noticed an immediate difference; 5 was the magic number. Good luck with it!