AU Vibration

[AUbigfoot]

Ok this is a fairly hard thing to explain so bare with me here. Lately when driving I have noticed an inconsistent vibration noise from my car and it seems to be coming from the back right. At first I thought it might be something like a screw loose on the number plate as that is the kind of sound it is making. I tightened all screws on the number plates, checked the spoiler as well to make sure it wasn't anything there. I have looked all over the car and cannot seem to track it down so I took it to a suspension centre for a look under it. The bloke there said that he has had the same trouble with his falcon and still cannot track it down. Everything under my car is fine and it hasn't got any issues. We took it for a drive and cruised to 60. Without breaking I down shifted to 2nd and after a second the rattle made it's appearance for about 2 seconds then disappeared again. I'm trying to pin point some other times that it happens, but the suspension bloke is in the same boat and can't pin point a specific time, location or driving style.

We decided maybe to post up on here and see if anyone else is having the same or similar problem as even he can't figure it out on his own car.
The ideas that we came up with are: bent heat shielding, air getting in behind something at the front and passing the noise down so it sounds like it's at the rear, something loose on the shell of the car.

I'm lost...