Why is my Explorer Sport making a piercing squeky noise when I drive? on 2001 Ford Explorer

Currently, when I drive my 2001 Ford Explorer Sport (4x4), the wheels (or something) seem to make a high pitched, squeaky noise every revolution. It seems to disappear at higher speeds. Also, it is not as loud when I brake. It happened every once and a while since I've owned for a couple years, but is now always making that noise (louder than it had previously). I have taken my car for the regular checks and have always given it TLC and when I have the local shop go through there complete check system, they don't find anything. What can this be?

Asked by for the 2001 Ford Explorer
It's important that the noise is occurring when the vehicle is in the shop being looked at. If you can re-create the noise consistently, take the mechanic for a test drive and demonstrate he noise for them.
Also, if this is 4 wheel drive, have the mechanic check out the technical service bulletin from Ford about a squealing noise from the front axle dust seals, TSB# 01-11-4. I had the exact same problem with my Nissan Pathfinder.
1 more answer
I would check the bearings the front ones these usually make weird noises when they go bad.