For the past year and a half or so there has been an annoying knocking noise coming from the rear of my 318is. It's a metallic noise and is in time with the car going over bumps on a rough road surface.
In an effort to find the noise I have done the follow:
Replaced the standard shocks and springs with Weitec coilovers.
The rear top mounts were replaced with Meyle HD units
All bushings replaced with OEM bar the four rear subframe bushings which are Powerflex
The handbrake shoes and cables have been replaced
The differential and both driveshafts have been replaced
The exhaust rubber mountings were replaced with OEM

So all of that has been done in the past year and have covered probably no more than 4000 miles.
In the past few weeks I've tried driving while holding the handbrake up, driving while pressing the brake pedal and driving while holding down the clutch and the noise remains constant.
Last weekend I stripped the boot completely of all plastics, carpets and sound deadening and the noise remains.
I also had someone drive the car while I layed back into the boot to listen for the noise and as much as I can tell it seems to be coming from the right side wheel well.

I'm open to any suggestions for what it could be as i'm pretty frustrated by it at this stage. Searching online suggests rear trailing arm bushings and rear top mounts but these have been changed.
So anyone who can solve this I will be eternally greatful