Squeeking Bump Sound From My GT

[2001GTPEWTER]

I've had my 2001 Mustang GT 5-Speed for 4 months now and I've put 4,000 miles on it (in addition to the original mileage), and just like with my 96 GT last year I'm getting kind of a quick squeeking soung from the rear wheels of the car whenever I go over a slight bump in the road. Its' not loud or repedetive, just noticeable when I drive with the window down. The transmission shifts fine and and everything else seems ok, but since this happened with my other one I figured it might be the way I drive the car (I run through those gears pretty fast and usually level off at a gear that I'm not at a high rev with. Does anyone have any ideas what it is?.. Its' like a split second 'squeek' and only from the rear wheels, maybe I'm hearing it wrong and its' just the tires rubbing sideways against the pavement (another clue that I'm starting to drive too fast again, I can't afford another ticket or FedEx will can me).