Knocking over bumps

[Simonbartlett]

So today I have investigated the knocking noise ive been having for the last few thousand miles. The upper arms seem to have small splits starting around the bushes but not too bad. The lower rear arm inside bush seems to be the worst so I was thinking of starting with them. Could anyone tell me the one part no. and manufacturer. It's a 2009 3.0tdi.

I have found a few but the measurements seem to vary, 80mm wide seems to be correct but diameter seems to vary online from 65 to 75 mm. The problem is im not sure if the tube stays on the arm and the center pushes out our the whole tube pushes off the ali arm ?

I have attached a pic if what I think is the offending part.
Thanks