First off, my apologies for contributing nothing but questions to the forum but I'm a new Audi dude and don't know jack about these things.
As for my 99 A6 2.8, if you can imagine the sound of someone sharpening a sword on a grinding stone, that's kind of like the sound it's making...or similar to dragging a metal can on the ground. Hard to explain, but it only happens when I give it gas, never when idling. Anyone experience this? No performance lags or anything, just a loud noise whenever I gas it.
Also, whenever I start the car after not driving it for 6 or more hours, there's a really loud fan that runs for a minute or two...it would seem to be like an "accelerated warmup" thing as I bought it in the winter, but at 70 degrees I don't know why it would do this...is it normal?
Scraping noise...is it only when you give it gas from a stop?
if so, I might suggest you check your rotor shields, which are metal. The sheild is on the inboard side of the rotor, and sometimes warps and scrapes against the rotors. Easy solution, just reach in there and bend it back so it does not hit the rotor. Just be careful if you hove just driven the car around a lot, the brakes are HOT!
Just a guess...good luck!
2002 A6 4.2 Sport...with a few things added