I explored some more tonight and I'm even more uncertain now about what it is, or what's causing it.

It's the exhaust note changing for certain - nothing mechanical is rattling or loose. The same criteria applies in testing...clutch in, slight revs (or slip the clutch as you would when climbing a slight incline) and it's there and impossible to miss. Not the sort of note you want your new instrument to blare.

The best way I can describe it is....flat flatulence. The exhaust note on these boxers is fairly unique as it is, and some have referred to them as flatulent to begin with. But to my ears, when it's idling or in neutral (or of course at full song), it's a symphony and harmonious. The flat tone while slipping the clutch is certainly not normal.

Also, it's a narrow window. Idle = fine. 2,000 revs = fine. Anything in-between results in our new little gal cutting the cheese...

So, I'll keep looking. I noticed tonight that relative to the other cars in the stable, she was quite drippy when cold...a bit of exhaust vapor condensation when cold (rich) is normal, but I had a small puddle pouring out the back of the exhaust tips tonight when looking once again. The other cars may bead a bit on the tips, but nothing even close to what the Boxster is producing. Curious...tank of bad gas perhaps, but we're on our 4th tank now and the sound has been there more or less since we brought her home.

No codes either (code reader handy) - so if it's a cat on the way out, or perhaps something with the air injection pump, it hasn't gotten bad enough for the car to throw a flag.

I'll post what it is when we discover it for posterity.