2013 Low pitched buzzing noise from rear when car is locked

I have noticed when going into the garage at night to charge the 2013 Volt, sometimes there is a low pitched buzzing noise constantly coming from near the rear axle. The only way I can turn it off is by starting the car and shutting it off again. The car is shut down and locked but the noise randomly starts after I leave it and come back later on. The noise continues until the car is turned on again. It sort of sounds like the mechanical sound the parking brake makes, but my parking brake is not on, the car is only in park and shut off. Has anyone experienced this? Thank you for any assistance.