My car has done this for a while now... normaly after a quick change in the road angle or dropping of a dropped cerb onto the road. It only does it once and is almost impossible to "make it happen" so im at a loss... The car has koni coil overs ( 4 year old though) its nothing to do with the exhaust as i changed that. I have adjustable rea tie bars ( 2 years old ) I was thinking maybe the bush's on the tie arms that havent been changed? or is there rear drop links to check??