We've seen quite a few of these now. It seems the 4WD 996 cars are prone to front wheel bearings wearing out. If you're taking your suspension apart and have a chance to try this, spin the hubs once the CV shafts are disconnected.

If it spins too easily, and especially if it makes noise, it's a good idea to replace it because the bearing is worn.

Here's an example of how it should not look/sound on a 996TT or C4/S. This car had about 30,000 miles. We've seen them on cars as few as 20,000, but it really depends on how and where it was driven.
YouTube - Bad front wheel bearing on Porsche 996TT