What causes a a squealing sound to occur?

What is your question?

My Jetta started making a squealing rattling sound on straight going. I seems to be from the rear of the car, but I'm not sure because I don't get the squeal when I turn left or right? I've had the left front wheel hub assembly replaced twice. The car has 108,000 miles. Brakes seem fine and the car stops straight without noise.