I have a brand new A3 2.0 and love it. but I have a strange thing happening. Shortly after starting the car, a strange knocking sound occurs coming from the passenger rear side of the car. It actually sounds like a small hammer is knocking onto the frame. It increases and decreases in speed in intesity but unrelated to the speed of the car, eventually it stops. It only happens after the car has been idle for a while (at least an hour). Its nothing in my trunk or anything loose in the car...its something mechanical. Any clues?

 

check the seat, and the trunk cover, anything inside the passenger cup holder......... seat belt maybe ?
but you said "eventually it stops"
no idea.