A dumb question, but was your stereo turned on at the time? I had a guy swear something was whining inside his car and it turned out to be a badly grounded stereo picking up alternator noise. He only noticed it when the power was turned on, but the volume all the way down. When music was playing, it drowned out the alternator noise. One day he just turned the volume down and left the stereo on, and that's when his problem "started"

I fully assume that this is not the case in your situation, but I just thought I'd point that out anyway.

-tp