Going downhill on my road bike with aero bars without applying brakes results in a terrible screeching coming from what seems to be the front wheel. Are the bearings going out?
Could be one of a number of things.
If you grab the wheel and shake it from left to right is there noticeable play? If you hold it by the axle and spin it does it feel gravely or gritty? Does it make any noise when you do that?
Are the brakes dragging on the rims? Is the rim buckled enough for the brakes to drag for only part of it? Is it worse in the wet?
Is it the wind in your ears, helmet or frame. Unlikely.
Are you going too fast ;)