FYI - answered my question and thought I'd post the answer for anyone searching in the future.
When doing a little more Google-ing, I came across a post by bar10dah buried in the "diary of a high mileage boxster" thread:
http://forums.pelicanparts.com/boxster-cayman-forum/601487-diary-1999-high-mileage-boxster-8.html#post6762570 (links to correct post)
Basically, the Fuel Evaporative Valve is supposed to click when you use the activation on Durametric.
I was able to change the valve today, and the new one does indeed click. When you start the Fuel Evaporative Valve activation it clicks about once every two seconds until you stop it. The click is probably 1/4 the volume of the carbon canister valve. You probably won't be able to hear it unless the engine is not running and the engine cover is removed from the car.
I reset the error codes and will update if the car throws the same codes again, but I am optimistic that this is it.