My car developed a faint drivetrain rumble around 20k miles when I tried on lowering springs, which resulted in a 4 degree angle on the axles, and the experiment was quickly ended with a return to stock springs, but the rumble has gradually worsened, now at 59k miles. The Norwegian member with a black car who also tried the H&R lowering springs experienced this problem as well, but is no longer active on the forum. PV1 says he feels it too, and IIRC, he's never lowered his car.
The rumble becomes noticeable under heavy acceleration and full regen around 35 MPH, and now mine is stronger at around 60 mph, while previously it 'smoothed out' at higher speed. On my car it is noticeable with or without a heavy load. Of course it's still less vibration than most any gas car, so both i-MiEV certified technicians that have evaluated the car claim they can't feel it after making miniscule adjustments to the tire balance. BS!
I think this is an issue of the CV joints being unsuitable for the suspension angles that our car is capable of. I've run much more extreme angles on aircooled Volkswagens with the IRS suspension and no problems (dune buggies, anyone?). The car should be able to run on the bump stops without the CVs being the weakest link... shouldn't it? Likewise, the CV joints are primarily affected by torque and axle angles, not like wheel bearings, shocks, or suspension joints. Even if the car was pulling against a stump at full throttle, the CV should not be the weakest link. An electronic component should shut off or cut back due to thermal overload long before a mechanical driveline component fails if the drivetrain is only receiving stock power levels. We ain't drag racing here.
Maybe I'll cruise by the Sonic tonight and ask some kid in a lowered Honda Civic with his hat on backwards......