Darn. It was the WORST CASE SCENARIO!
Yes, it was a FALSE ALARM and I POSTED IT FOR ALL THE WORLD TO SEE THAT I AM A BOOB... (i.e. my mistake)...
A dealer tech listened to it and said it was normal.
It's simply the sound of a Boxster's engine between 2900 and 3900 rpm...
I guess with all the top down driving, I forgot that it made this whine.
And I even posted the wrong symptoms. i.e., it makes the same whine regardless of whether or not the engine is providing forward thrust.
The reason that pushing in the clutch stopped the noise was that I also let off the gas and the engine RPMs fell out of the 2900-3900 range.
Very embarassing.
It's most noticeable at about between 2900 and 3900 rpm. If others could say their Boxsters do the same thing it might make me feel better.
Sorry for raising a false alarm.