2004 Audi A4 1.8T

I am not sure of the answer to your question, but you are not alone. I have a 2003 Audi A4 1.8T with 60,000 miles. I have a similar humming or vibration noise coming from the car. The noise seems to be coming from the under right side of the car, and I am suspicious of the exhaust system also. I just had my car in the shop, and the garage could find the source of the vibration. The humming sound still continues, and seems to be worse when the car is idle or at low speeds (city driving) rather than highway driving. Did you find the answer to your issue?

I took it to the dealer to try and figure out where the noise was coming from. The vibration sound you're taliking about (which you clearly hear during normal street driving) seem to be a brake problem. The dealer told me (after 4+ hours of test driving my car) that my right caliper was loose and that my after market brakes could have influenced the problem. The dealer tighten my brakes and since then the humming has stopped. It could also be noise comming from your brake shield; if your car is lowered it too can play some part in some unusual noises.

I was wrong about the exhaust system- I took mine to a muffler shop and they checked for holes and nothing was found; they did needed to tighten my pipes in th center of the car.

The other noise(s) I'm trying to figure out today is:

Why is it when I hold up the window buttons it sometimes makes a humming noise? This noise happens every so often.

Why is it when I change gears (car is stoped going into reverse or into gear) that when I press my brakes I hear a metal ding noise?