No..... what?

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Chapter 3. Still trying to diagnose this. My Porsche mechanic made a house call, and spent the better part of an hour trying different things while the car was up on jack stands, rear wheels off, engine running, and in various gears. The noise is intermittent again, but he tried various things to get it to act up. He heard and saw some things. He does have some theories, some of which are big (transmission) and some pretty minor (axles), as Porsche stuff goes. We do know that the left side rear axle has a bit of a wobble, but the sound is on the right rear. It's going to be flatbedded to his shop when he has space on a lift, probably in about 10 days. He's been a Porsche mechanic for over 40 years, first for a dealer and now at his own well-regarded indy Porsche-only shop, and he's never come across this combination of sounds.

Keeping my fingers crossed.

Well, a little time has passed, and I've been driving my car since the repairs. We replaced the axles, and while my mechanic was doing this, he found a "failed keeper holding the c/v on the axle letting the axle bang the end of the output flange." At least that was the really loud noise heard just before we flatbedded. He's still not sure what caused the noise we first heard on the trip. We know that the stupid mudflap was flapping around, and taping it immediately quieted that noise. In addition, the axle was bent, however that happened, probably before I bought the car. But eventually the clip failed entirely. The interesting thing was that the sound couldn't be duplicated while test driving at the shop after the flatbed arrived. Apparently the lubrication had to be thoroughly warmed up--30 or 40 miles or so--before the noises started. Weird.

So, all of you who voted for cv joint issues, you were right. It was just a bit more complicated than that. I've put a few hundred miles on the car on a variety of roads and at various speeds. I haven't heard any strange noises aft of the cockpit, and the bad vibration at higher speeds is gone, too. Smooth as a baby's bottom, in fact. Thanks for all your help and your long-distance guesses!