This was my previous post:
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

Now I am positive that it is my transmission causing my problems. Because last night on my way home from the drive-in my car was making a thump whenever it shifted gears. It was most noticeable
when it shifted from 1st to 2nd. 2nd to 3rd wasn't as bad but it sounds more like metal hitting metal. And to top it all off, when I went to get my car out of the garage this morning there were 3 small puddles of tranny fluid on my garage floor. So, I will be calling up the transmission shop that installed my converter to have them take a look at it. I just hope it won't cost too much.

 

...are you running long tubes? I have the same thumping and it's the y-pipe hitting the floorboard

 

transmission mount or torque arm bushing check those first