It seems to have many names but my car has a knocking noise coming from under the car when going over rough ground and this seems to be the fault.

I've seen that the rubber damper has some movement but is not loose.
When you tap the rubber damper, in the direction of the propshaft, with a rubber mallet the knocking noise can be heard.

I've had the propshaft quickly inspected and there isn't an obvious problem there.

Has anyone had this problem before?

What is exactly causing the knocking noise?

If I was to cut off the rubber damper would this fix the problem?

 

I had the rubber damper come completely loose on Theseus. That produces one hell of a vibration at 140mph. You can remove it, but you will hear more drivetrain noise, particularly on decel. Removing it requires cutting it off carefully or removing the pinion flange, and removing the pinion flange requires resetting the pinion depth (essentially a full rebuild).

 

My damper is loose too. I can check if it makes a knocking noise later.

 

Thanks for the replies.
@aidandj The noise is like my exhaust is loose and is banging on the car when I go over bumps. It's quite loud which makes me think that it's more than just the rubber damper being loose.

 

Finally got my car to a diff expert.
Guess what?
It wasn't the companion flange after all.

The drivers side rear caliper bolts had loosened (one had fallen out).
They replaced all the rear caliper bolts and Loctite threadlocked them.

You live and learn.