560 SEL suspension knock
What you describe sounds to me like the strut rod mounts--which you have replaced? Or the big rubber mounts for that big aluminum alloy mounting housing are bad and letting the mount hit the frame. I have a pair for one side I'll sell for half price if that is what it is.
It also could be the hard rubber mounts on the front end of the strut rods.
Keep looking till you find it--I wouldn't feel comfortable driving it until I knew what the cause was and could judge the safety.