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
2 answers
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.
I would check the bearings the front ones these usually make weird noises when they go bad.