Does it really bother you that much?
Mines done it since the car was about 6 months old, just never really thought much of it.
It's just a zing and gone in half a second.
But just to check, look up the nearside exhaust pipe and get someone else to start the car, the flap (easily visible, only 2 inches up the pipe) will be OPEN before you start it and it will close when it cold starts, then it will open after fast idle (about 30 seconds).
See when the rattle happens, i.e. during starting, or closing or opening etc.