There is a deep thumbing sound come from the rearend area from about 15mph and up, it only does it if your on the throttle, if you let off the thumbing goes away, it's a continuses sound when on the throttle. It a 96' TA WS6 A4 90K, 3.23 gears. if anyone can help just let me know.
thanks
chase

 

whens the last time u changed out ur diff fluid?

 

i bought it with 88k and i have not yet changed it, i don't know if the previous owner ever did. if i do is there anything i can find out about it by the color of oil or anything like that.

 

If you pull the cover off and there is metal in the oil, thats not good... Or if you find big chunks missing from the ring gear.. that will tell you real quick. Probably a good idea to change the oil, and if you don't find anything, look else where. Sounds like you're going to find something though.
Just use a little common sense.. And don't forget to put friction modifier back in when you change the oil.