I was meaning that a consistent tapping/clicking is likely to be the inner CV joint (slightly worn) BUT many of them seem to do it (replacing it with another used one and cleaning/regreasing made no difference in my case.)
The joint seems quite loose when assembled dry, compared to a new CV joint that I bought for a Honda last year (or two for a Subaru this month), which were all very stiff to move.
I still believe that cleaning/regreasing is essential when you notice a split boot (to extend the CV joint's life), but I think once a light clicking has started, you might as well fit a new CV joint if you're going to the trouble of removing it. Hope this clears up what I meant?
Alternatively the light clicking/tapping is tolerable if you can't hear it with the window closed (I've tolerated it for years). I'm sure when greased-up it will last for a long time. The outer CV joints are more subject to wear and make a much louder noise when worn.
Meanwhile it would be funny if it was something bizarre like a loose spring in the clutch plate, but I think you'll be able to figure out whether it's road speed or engine speed related. Also, if it's a driveshaft, it's generally more noticeable when turning a corner, but in cars like the Spider the driveshafts tend to operate at a set-back angle even when the wheels are straight ahead - so even this last point isn't a definite identification.
Sorry about the uncertainty but I think you're on the right track - check for stones in the calipers first!
also brake pad anti-rattle springs...