It *could* be from the exhaust. Were you able to tell roughly where it was coming from (front, back, or middle of the car)? When I first installed my Fabspeed x-pipe and HFC, they were rubbing on the trans cross member and it sounded kinda similar. Maybe your header gaskets have failed allowing the exhaust to rattle around until it heats up and seals.
2008 IB E92 M3| BBS | KW | Arkym | Platte Forme A.G. | Active Autowerke | K&N | Fabspeed | Dinan | Evolve-R