Groaning sound from rear of car

I've noticed a low groaning sound coming from the rear of my 08 veracruz (73,000 mi). It's not connected to the engine or the transmission- the pitch of the groan changes w/ the speed. Had the shop at the school I teach in take a look and they couldn't find anything wrong w/ the rear end, but they can't take it out for a test ride. I have an appt. scheduled w/ hyundai. About 1.5 years ago I had a bad bearing and hub assembly replaced back there. Good thing I purchased the extended warranty.

Anyone had any experience w/ this?