It could be the fans on the rear of the refrigerator. Get a mirror and light to view the fans and verify if this is the hum you are hearing. The thermostat might be going south as usually you have a 20 degree difference in cut in and cut out temperatures.
Mike, RVIA & RVSA Certified Master RV Technician
Amy, Dr. Assistant - Roxie & Mei Ling, four legs each
2000 Gulf Stream Scenic Cruiser 450 hp & 1330# torque
06 Saturn Vue, 06 Chevy Z71 4x4 & 2014 Corvette Z51 M7