Toyota Tacoma that hums when it's parked

I purchased my Tacoma used in 1998. It is a 1997 model, 4WD. I soon discovered that a humming sound that is bit like the buzzing of bee’s wings mixed with a low mmmmmm. This sound is coming from the engine and is always doing it when I check. The truck runs well. Does anyone know what might be causing this?

Maybe the gasket around the radiator cap is worn. Did you open the hood and try to locate the sound?

Fan running?

I need to give more info. We are gone from home for weeks at a time. It will still be moaning when we get home after being gone four weeks…even more. There is a black box in the engine compartment behind the grill on the passenger side. It was summer time, the truck hadn’t been driven in probably a month and was moaning faster than ever (Arizona heat). The sound seemed to be coming from the black box. I slapped it with my hand and the noise stopped…for a while. Thank you.