Mine does this also, and it seems to be down to one of the spurs that hold it in place - the little rubber sleeve has come off, meaing it rattle like buggery over bumps. Haven't tried fixing it yet (I just turn the stereo up louder - I had a Fiat for 4.5 years, I know all about masking rattles

), but I reckon some Dynamat round the spur should fix it. Just need to get round to doing it :/