This problem is now resolved, and I’m a very happy guy.
After failing to identify where the exhaust was rubbing on the chassis, I built some wooden stands to get the car high enough so I could have a proper look. The car fell off my jack stands a couple of weeks ago and I don’t feel good about using them for any lifts beyond 6” or so anymore. Thank heavens, there was no damage to the car. Oh, and I’m ok too.
A close look quickly revealed no contact between exhaust and chassis. Hmm, weird. Motor mounts are tight, too. Can’t be the exhaust after all, it seems.
So I refreshed my memory on the prybar-as-stethoscope trick on this forum and on YouTube, and went at it. A couple minutes later I had traced the source of the vibration to the sump, getting more and more depressed as I honed in on it.
Then I noticed, “the sump does get kind of close to that transverse chassis brace (the one painted in body color, on all cars I presume, or is it just mine?) — maybe there’s some rubbing there, somehow, even though the relative positions of sump and chassis couldn’t really have changed.
Crawling right under there, I start looking. Plenty of clearance. Hey, what’s that? Motherbrother.... the missing exhaust stud is wedged solid between sump and chassis brace, transmitting engine vibration directly to the chassis.
I wrestled it out with some needle nose pliers. It was really jammed in there.
Start ‘er up, never sounded better, winds up smooth as a turbine.
Thanks to all who gave their technical and moral support!
, your point is well taken. On replacing the missing stud I discovered another one that was very, very loose. Both are now replaced with original studs cannibalized from my old cylinder head. Lesson learned!