Hi Mark,
I think we've all spent ages working thru the kilos of rubber bushing under a merc, trying to find the place that one last annoying thunk thump thunk really comes from.
I was just about to swap out the guide rod mounts when I noticed that it was the intermediate arm bush (you know, rubber at both ends, sits inside a steel tube on the other side of the car from the steering box..) that was the culprit.
What I learned is that you need 2 people (one to move, the other to watch) when looking at this stuff. I'd tried alone many times with no joy; did it with a freind and we quickly found the real problem. Hope u do 2
Giles