I went to a local 1/8 mi. track last night with my '98 SS A4 and when I took off from the line I could hear a thump...thump...thump...thump...thump sound coming from underneath the car. It sounded like someone was pounding on the bottom of my car. It seemed to come from the middle to rear of the car. It only does it under hard acceleration. If I ease the peddle up to 30mph and then floor it, then it's fine, no sound. It's only maybe for the first 5 seconds from when I floor it until it gets up to speed. It's not my transmission mount because that was my original guess so I bought an energy suspension mount, even though the stock mount looks fine, and that made it worse, plus I could feel a lot more vibration from my transmission. I know it's not wheel hop because my car doesn't shake or bounce at all. Also, my engine and whole exhaust system seems to shake more than usual at idle and it makes a slight rattling sound at very low speeds that smoothes out with increased speed this might be related to the thumping noise. My only guess would be a bad engine mount. Any help is greatly appreciated.
Jason

 

It's possible your exhaust is hitting the bottom of the car...only thing is, I would think it would do it continuously if it was the exhaust. What kind of mods have you made to the car? Any headers, aftermarket y-pipe or exhaust? If so, it'd probably be a good idea to get under there and check things out. I'd make sure there's at least a good 1/2 inch clearance around the system. Anyone else have another idea???

 

I have slp loudmouth and y-pipe. Also, 3500 stall converter.