2005 Pontiac Grand Prix car noise

My car is making a high pitched squeeling noise that isn't there when I leave in the morning for a 45 mile one way commute, but on the way home it starts right up and doesn't stop, it pauses when I swerve to the left or right, I got my brakes replaced, front an back, I also had a mechanic check my wheel barrins, he said they seem fine! My tires are not that old, and the only thing that needs to be replaced on my car is an O2 sensor! It seems to sound a little bit like a brake pad when its wore to metal on metal.
please help me!