Been chasing a very slight knock on my E92 M3. It happens when you go over a bump, but one wheel at a time (ie. not squarely) which then causes the car to rock side to side as each wheel goes over it. I could replicate this by just rocking the car side to side when stationary.
Checked everything that I could think of and all was tight. Checked the ARB links were bolted up tight but it was still there. I was suspecting it was the droplinks so was going to order them anyway, but I wanted a proper diagnosis, so I decided to remove the front ARB drop links and see what the ball joints were like off the car:
Needless to say, they need changing! All four ball joints had excessive movement, all four have split boots, with one of the ball joints having the slightest bit of play when pulled back and forth when off the car. Would have never diagnosed this on the car, so i'm glad I got them off!
Ordered the fronts and rears as I'm sure at nearly 100k miles, the rears probably aren't great either.