E30 Zone

Hi a friend of mine has a e92 330d msport with a knocking front left suspension...garage can't find anything wrong....any ideas before he starts shotgunning parts into it....
Many thanks

Wasn't on an E92, but was on a BMW with alloy struts: The steel damper rod had worn the hole in the top of the strut to an oval shape nearly twice the size of the rod. The whole thing had no damping effect whatsoever, but the car drove seemingly OK, apart from the knock, and had passed at least one MOT test in this condition.
The problem wasn't really diagnosed until the spring had been removed from the strut.

Brians right. It'll be a worn damper or the top mount has given up. Unfortunately, BMW's sport suspension settings hate speed humps, pot holes and the general shite state of UK roads. All of mine that have had sport suspension have needed comprehensive work due to where I live and Surrey councils refusal to tackle the road issues full on.

Thanks guys I presume it will have to be taken apart to diagnose properly? Can they be changed on their own or do both sides need doing??

If you take the wheel off and drop the hub onto a secure stand, so that the spring is compressed to its normal amount, you should be able to feel/see if any lateral movement occurs where the damper rod enters the strut when you apply sideways pressure.
It's always recommended to change dampers in axle pairs, as it is for springs, brake discs, and suspension bushes, but if the damper is as worn as the one I came upon, then it's going to be less unbalanced with one new damper and one serviceable old one than it is now.