What could the source of a constant electric hum in my apartment bedroom be? I'm on the 2nd floor.?

Question

The wall is the one that faces out toward the street, next to a window. I thought it might be related to the baseboard heater but the hum definitely is coming from behind the wall - you can hear it best when you put your ear to the wall.

No other wall has this sound.

Answer 1

Shut off all power to your apartment.  If still present it has to be another outside source, such as a noise coming from a pipe in the wall transmitting sound from another apartment.  If it is an outside wall is there a source outside the wall such as a power pole with a transformer hanging on it?  The more power is used from a transformer the louder they hum.  A window unit air conditioner near you could also be transmitting a hum especially if your apartment was built with metal studs instead of wooden 2x4's

Answer 2

window ac unit on the same wall above or below you.  it is not as nosiy now and you here the vibrations