If you have a nasty squeak/groan noise right under you when you go over bumps, its the sway bar. What's causing that noise is the sway bar sleeves. They tend to slip and cause squeaky noises. The newer sway bar, Audi has removed the sleeves and its just the bar and bushings. Hopefully this helps, good luck.

 

Ok so I read on your posts that alot of people have this issue, and you're probably tired of hearing it said, but I too have it ('02 model). It sounds like its coming from rear passenger side, kinda like right around the fuel cap. How can I check and make sure it's not the sway bar as some have suggested? I am the farthest thing from a mechanic, so hopefully I can make sense of your suggestions.

However, I also hear another thumping like that, but louder, from what seems to be behind the passenger airbag. I noticed one day that I could force it to happen by turning the A/C on and off really quick, but that doesn't always work. The Audi service guys said that it was something like a trap door in there opening and shutting, and that it couldn't be fixed.......buuuuuuutt they can be kinda dumb sometimes. Anyone else know about this?