[Steve Tinker]

If I'm reading this right, you are able to push / pull the strut sideways by .035" (I can't see where your dial gauge is mounted to).
The strut shaft is bolted to the upper strut mount by the M14 central nut and the strut mount is bolted to the chassis by the 3 nuts you say are tight. The strut mount itself has a bonded rubber insert which can and does break away with age. Not an expensive item, but quite a lot of labour to replace.
The only other component up there is the strut bearing itself which can rust and wear and cause a knocking on rough roads. I'm willing to bet that these are your culprit. If you have the Bentley Boxster manual look @ page 40-5 for details.

Using a pry bar on the suspension bushes will reveal nil movement, even when they are worn. With 75,000 miles on your wishbones / drop links etc., there's bound to be some wear which shows up as a knocking or rattling over rough surfaces.

I replaced my 50,000 mile S suspension with PSS9's a few years ago and fitted new top mounts too as a precaution - the old ones looked OK & the bearings were fine, but I live in a warm dry climate which always helps....
Pity you don't live closer or you could have my old suspension & mounts cheap!