I'm sharing this in case anyone else gets driven nuts like I did. This is on a B300 chassis but I guess could apply to others too.
A little while ago a loud knock/rattle appeared under the front pass. side - sounded exactly like a loose caliper but it was most noticeable when applying the brakes over an uneven road (applying the brakes generally makes loose brake parts go quiet). I looked at everything I thought it could be and i couldn't find anything.
That included me checking the upper ball joint by rocking the wheel top and bottom - nothing, it seemed just the same as the other side.
So I finally took it to my friendly garage guy who put it on his lift which has a hydraulic suspension testing set-up (it rocks and jiggles everything for you while you look underneath to see what's loose) - still nothing !!
By this time I was starting to think I must be hallucinating.
Eventually we decided to try driving it onto the brake testing rollers, and pressing the brake pedal with the rollers turning we found it - the upper ball joint is worn out, but only in the forward/back direction; not up and down, not side to side.
Really hard to find - I guess with three people (one pressing the brake, one trying to turn the wheel, and one checking the joint) I could maybe have found it without using rollers. Anyway - at least I know everything else it tight