soft humming noise.

Hey all. So I've got this 1987 Audi 5000s quattro. Great car. Newly swapped leather interior is extremely comfortable.

Breaks stop this car like it weighs nothing and the little 2.3 litre hauls it around pretty decently. And I can't say enough good things about the quattro system.

Now the problem. Not really a problem really. Just a musing. I have an extremely soft humming noise at all times. Varies with speed and absolutely nothing else. I take hard corners (took it autocrossing even) with no difference. Can't even hear it when at low speeds. At highway speeds it is very soft.

So I've heard that the wheel bearings go out every 50,000 miles. I've had wheel bearings go out on me before in my rabbit. When they went out they were very noisy and noticable. This sounds absolutely nothing like that.

Does this sound like a problem to you guys or is this a normal sound from the all wheel system, the tires or the fact that all the tires are trying to turn at very similar speeds?