When I drive the car at low speed almost to a stop, I can feel like the back wheels (it could be only one of them, it's hard to tell) are going over a small bump or another way to describe it is like a large pebble stock in the tire. I looked at the tires and I can't see anything wrong, the car is a sti 05 and has fairly new winter tires on it. Any idea what of what it could be?