I took some videos, first the engine idling under the hood:
https://youtu.be/twWLpcObpDM
Secondly inside the car where you might hear the booming kind of vibration:
https://youtu.be/s2KBrQkcs3k
Another thought I had this morning, I don't remember this vibration being quite as bad before I changed the engine mounts, it did clunk more as the drivers side engine mount was failed and leaked all its fluid, the passenger side one was ok but I changed it as it was only a matter of time before it failed too.
Perhaps the new Febi mount on the passenger side is defective or just poorer quality than the OEM Benz one I took off?
I should be able to tell underneath if the mount is transferring the vibes through to the body.
Incidentally does the engine idle under the hood look good? to me it seem to idle fine, its inside the car the problem seems to occur.