My 04 A4 Quattro just started having this mid-pitched buzz/hum/rattle from the rear of the vehicle. It isn't very loud, just sounds like somekind of metal plate or something is loose? The noise is there during idling and does not change pitch or volume when accelerating. Anyone have any ideas???

 

Buzzing sound in the rear is usually a fuel pump. But the metal sound!!??? I dunno. Have u looked underneath your car? maybe something is loose?

 

I guess it had something to do with a fuel filter. Apparently some models/kinds are faulty? Either way, that solved the issue...