Humming sound from back and lower rear of vehicle

Since I cannot hear it, and if it occurs when the car is standing still, the best suspect is an exhaust leak. The system needs to be inspected to see what is needed.

Another cause of humming is a rusted or broken exhaust pipe hanger. If an exhaust system part rubs against the car's floor or other part of the body, the noise will be transferred into the passenger compartment, and that can be rather obnoxious. The clue is you will usually hear clunking or banging and the humming will stop momentarily when the exhaust system shifts as you go over bumps in the road.

Also consider the electric fuel pump in the gas tank. The clue to that is you will usually hear it hum for one second when you turn on the ignition switch, before the engine is running. It wont resume running until the engine is rotating, (cranking or running).