Did you ever find out what it was? I have a '12 CTS and between 45-60 mph (not sure of RPMs) I get a deep, low hum coming from the rear end. Car has 45,000 mi. I remember doing a test drive a while back and the test car had a similar hum...must be something with this model?
I'd suspect the diff/fluid too. I have a Chevy van that has a noticeable hum from ex: 45-55 MPH that disappears quickly above that speed. Looking at the shop manual one time I noticed they are actually designed that way i.e. the inevitable harmonics/resonances are designed to occur at less frequent/transitory speeds. There's a whole process/description in the manual about getting the hum speed/tuning right when working on the diff.