Thanks, Scott, those are some great ideas! The vibration that we feel (both driver and passenger) can be felt in the seat as well as the steering wheel. In all cases, the vibration occurs at approximately 65-70 MPH. It always occurs in that speed range and under acceleration, primarily while driving into a slight incline. The difficulty is that the vibration only happens half the time I try and reproduce it, even on the same stretch of road. In fact, Tesla thought the issue was completely resolved with the DU and rear axle replacements, as they were unable to reproduce the issue afterward until I drove the car. That's when I was able to make it happen for them.Hmm..
Passengers feeling noticing this in front of the car. ..
Just wondering if it's not some accessory under the frunk bin... like A/C compressor or brake vacuum pump.. something that cuts in and out. If you have a vac leak that pump will come on at random times and will work hard (long) to make more vac as she's leaking... hard time catching up. If the vac pump is rubbing against something or its bearings are starting to get upset... vibration..
When "all is quiet" on the freeway... do some long light lingering braking pedal presses (to drain the reservoir) and see if the noise starts up!
I found the A/C compressor hoses rubbing on the bottom of my frunk bin, and I'm sure the contact and vibration was amplifying the sound of A/C compressor when it was working hard. Started to sound like a monster under there on the hottest of days. (Will need to wait until next summer to see if its gone.)
I'll mention your suggestions to them and see what they think. The service foreman is leaning towards it being a rotational issue of some kind. He also noted that it seems to occur most often under acceleration load, or in other words, when the weight shifts to the rear and the car squats down. Other than throwing parts at the problem, they don't really know what else to do at this point.