Act of God

ok, I have a 2001 S4 with 53,000 miles on it. It is a tip (yeah, i know)

Lately, when I am driving the car it sounds almost like the car needs to shift. I look at the tach and its low around 1,500rpms. Even if i put the car in neutral the sound is there.

It usually happens around 40mph when I am laying off the gas and just kinda coasting. The sound is almost a whirring noise. Like i said, like an engine humming along at a higher RPM (not red line screaming).

The noise sounds like it is coming from the rear, underneath the car. Any ideas?

Dubs

It turned out to be a pinion bearing going into the diff. I got the pinion and carrier bearing replaced. Not sure if it is the same in the S, but it was a constant hum that increased with speed not RPM. Try listening for the sound on turns, because that might hint to something along the lines of a wheel bearing. I'm no mechanic, and not really that competent with Audi's but that sounded like my problem.

Act of God

thank god it's audi assured! It definitely goes with the speed. RPM's are completely unrelated to it