Noise under the rear of my ford explorer?

i just bought a 1998 ford explorer limited 4.0 engine. when the car is running and when its shut off there is a noise coming from under the rear end. sounds like a buzzing or humming sound, almost like a small air pump running. runs about 10 seconds at a time and does it 3 or 4 times after the car is shut off. it sounds almost like a caddy with the automatic leveler but i don't believe this has it, nor would it run a few times after the car is off. its below the cargo area, right above the spare tire. its a black rectangular box with 2 wires or hoses coming from it. looks like there might be 2 black boxes. i have no idea what it is or why its running after the car shuts off. everyone asks me whats making the noise and i feel dumb for not knowing what it is. please help.