I had no idea there was so much going on inside one of these! See the section half way down the page:
https://*************************/pr...15428796317786 Damn censorship! w h i t b r e a d p e r f o r m a n c e . c o m
Thanks for the heads-up that they're a good alternative to OE.
I'm getting a quiet but definite knock around 1000-1500rpm. I can only hear it faintly when driving on smooth road surfaces, but it's definitely there and if anything it's getting slowly worse. Is that likely to be caused by the damper going bad? M57TU2 330d, 138k miles.
Also, I remember from putting the stretch belt for the HPAS on mine (~10k miles ago) that it seemed excessively tight. I'm confident it was the right part, and was either from Conti or Gates (can't remember)...could that have contributed to early/accelerated failure?