squealing noise - what could it be?

My '99 XJ was making a new and weird noise this morning on the commute to work. I'll check it out when I get home, but would welcome any guesses as to what it might be, so I'll know what to look for first.

You know the squealing sound a tire makes when you take a corner in a parking garage with a smooth concrete floor and lots of surface for sounds to bounce off of? My rig is making a noise just like that, even when I'm going straight ahead. With the window open, I can hear it even if I'm not driving alongside a wall. It's definitely coming from a wheel or tire, and not from the engine compartment. I'll check the tire pressure, but the tires all look fine, so I really doubt that has anything to do with it. Any other thoughts?

check your belts

is the a/c on or off?

if its coming from wheel area maybe it's the warning tab on your brake pads or a brake shoe that the lining is low. Also could be the backing plate rubbing the rotor.