As FB said, fretting is likely the issue. There's no lube between the bb and the race, where it is needed. It gets hammered out by impacts on the front wheel while riding straight ahead - and this is most of the time. Periodically rotating the fork 360° (one full turn) can help. It repositions the bbs on the race, and spreads the grease in the headset.

That's my theory, and I'm sticking to it. I am not a pro bike mechanic.