Some good suggestions and feedback, thank you.
The car has ~53k miles, but I've only owned it for the last 3k miles. The tires have maybe 1k miles on them. The old tires were worn out, noisy, and harsh - - so ironically, I think they masked the vibration. I really noticed the vibration after installing the new tires. After having the rears rebalanced twice, I had TireRack RMA the rear tires. My installer for the replacement tires is a well known race shop in town - - they said the new tires/wheels balanced up perfectly in the road force machine. The wheels are in nice condition and not obviously damaged but I've not specifically had them measured for run-out.
Some additional info I've been withholding so as not to overly bias the responses:
When I was changing the rotors (to rule them out as the vibration source), I ran the car on jack stands (at the jacking points with all 4 wheels in the air) with the tires/wheels mounted and not. The right rear wheel seems to have the most vibration irrespective of whether the tire/wheel was mounted. It also looked like the right rear axle was "jump-roping" just a bit, i.e., swinging around in a larger circle versus spinning perfectly on axis.
With the above additional information, Google searches suggested that bad CV joints could cause strange vibrations/axle alignment issues, so I thought maybe that was my problem. I bought a pair of low-mile used axles and rebuilt/greased/booted them. So far I swapped the right rear axle and there was no change. Maybe my issue is left axle (
TBD), but I was pretty bummed when swapping the right one didn't fix it given the jump roping issue I observed above.
Regarding the "beating" vibration suggestion - - two vibrations that come in and out of phase causing the vibration to cycle - - that does make sense. I'm open to suggestions on how to test that. But it doesn't explain why the vibration is more pronounced on grooved pavement.
Google searches suggest that the wheel bearings on the water cooled Porsches (996/7s and Boxster/Caymans all share the same bearings) do fail. But most cite a noisy growling noise, which I'm not hearing. Many have reported that it's difficult to feel play until it gets really bad with the bearing type used. There's only a handful of reports of bearing issues without noise, but there were a few report that also had weird handling issues. A bad bearing in the front appears to be more obvious (and frequent). Rear failures seem harder to identify.
I've been thinking wheel bearings based on my vibration readings, but I don't want to blindly throw parts/time at it.
Suggestions appreciated.