Rear humming noise at 60mph manufacturer defect?

[c2thew]

My dad's vibe hums at 60mph and then dies down either below or anywhere above that speed. He came to the conclusion that it was the muffler's harmonic frequency that causes this hum. We've never noticed it before until about 5 months ago.i've done some light reading and it seems to be either 1) differential, 2) exhaust pipes or 3) bearings.am i correct? if it is the exhaust pipe, is there anyway to secure it so that it stops humming? I get super annoyed as I have to sit in the back seat and listen to it as he constantly cruises the car at 60mph to "conserve on gas"

[Caretaker]

Is your car an '09?? The 1.8 liter '09's all have a harmonic sound coming from the transmission between the speeds of 60-68 mph.

[c2thew]

i think it's a 2006