Has it always been that bad? How about with the car turned off? AM is very sensitive to electrical interference, so make sure the antenna wire is grounded properly at the radio and at the roof mount. A bad antenna might seem to affect AM more because FM reception will still sound pretty clear, just not have very good range. There are lots of things that could cause interference in your car, like the ignition system, alternator, even fuel the pump.
If it's still bad with the engine off and the antenna is grounded properly, the problem is most likely the radio itself or a poor antenna. Maybe you could try a different antenna (even a temporary non-OEM one) and/or trade radios with someone as a test.
Then again, my experience is based on normal cars, not Audis.
