Wheel Squeal

[Crazygirl]

My daughter has a 2007 Grand Prix GT Supercharged. Recently we had the rack and pinion replaced because it was leaking horrifically. Luckily, it doesn't appear that the power steering pump is bad.
However, we also replaced the brakes, and the rotors because the previous owners literally drove a groove into the back of two of the rotors. The front left tire had a slight wobble, and my hubby suspected the front bearing was bad, too. But he took it all apart, and said the bearing is fine.
Here's the thing-my daughter can still hear a slight squealing sound coming from what sounds like the area of the front left tire, but mostly when she's backing up, not so much going forward. My hubby is stumped, as it doesn't appear to be a bearing, the tie rod, the rotor, or anything else.
Would anyone have any thoughts on this? I don't want to dismiss it as nothing only to have her stuck on the side of the road with a wheel coming off or something. Thanks!