My car, 2020 Toyota Camry, has been parked and engine off for the last 10 hours. The left rear tire area was making a low humming/vibration sound. The gas cap is directly above it, if that matters. It only lasted about 2-3 minutes. I've heard it at other times, too.

What is it, please?

 

The car has a system that checks the fuel system (fuel tank, fuel and gas fume lines, emissions system) for air leaks. It uses a small air pump to do this. If the system has an air leak, a pressure sensor will detect the leak during the test. The check is typically set up to be done when the fuel tank is between a certain level. The engine then needs to be off and the car has sat away before the test is done.

The noise could be this test being done.

If the test finds a problem on a consistent bases, the check engine light will come on.