Driving home yesterday, I experienced a horrible, intermittent metal-on-metal grinding noise originating from somewhere on the rear underside of my new 2000 323i. The sound lasted only a fraction of a second each time it occurred, at a frequency of once per wheel revolution. It only lasted about a mile, and disappeared following the exit of a 90 degree turn. It didn't sound like anything lodged in a tire and I haven't heard it since. I don't think it's anything I can have checkout out, given that it has discontinued. Any possible causes (bearings, differential) or easy checks? How concerned should I be?