There has been a low soft sounding humming noise in my house possibly from the roof or a pipe-Soft whirring sound-  we can not locate it. It is not electrical since it persists even during a power out. We thought it might be bees but we don’t have an attic( a frame house built in the 80-’s). I thought it was from the roof, perhaps the vent pipe( there is no roof vent)but I hear it in the basement too. It drives me crazy when all else is silent and I hear this low frequency sound that we can not locate. We thought it might be in the pipes ( it sort of sounds like a bathtub draining, but very soft Low sounding version of that) . When I go outside the house I do not hear it. 
Any suggestions for how to resolve this?