Noisy rear

Wondered if anyone can help me. I have a 1998 740i which has generally been good although appears to be going wrong (needed new rear suspension / Radiator / expansion tank/wheels in last 4 months)

The latest is a noise from the rear of the car which sounds like a low thud. There is no pattern to it and it sometimes happens when the engine is off??

Can anyone help?

I have the same car. From other boards, I understand this noise can be the gas tank. Try googling that.