What are the causes of this?
The engine runs fine and very strong, its done this since bought 3 years ago.
AFAIK it hasn't had any aftermarket engine mounts.
The mount that is attached at the bottom of the firewall (below the turbo) is prone to stripping its threads and the bangs against the cabin. Try tightening the bolts up but think you may have to get a helicoil kit out
Good call, Johnny
Just an outside thought but I know that if the exhaust flexis, bushes or brackets are worn it tends to drop onto the subframe and the vibration can propogate through the chassis into the firewall. As this is an easy visual check, it's worth taking a look.
John.