Audi: Weird sound coming from wheels (?)

I have a 2003 A4Q-6M, and live in a cold climate (N. Illinois). In the
last couple months I've noticed a new sound coming from the wheels, but only
when the car is cold, and only once I get above about 25mph. Once the car
has been driven for several miles, it seems to go away. The sound is kind of
like a "squeaky" wheel bearing or hub (although I doubt that's likely). It's
pitch is "mid-range" (not high, not low), and it doesn't really vary much
with car speed or engine speed. It's definitely noticeable. Any ideas on what
it could be? Has anyone else noticed something similar before?

Thanks,
Bob