i had this in my old house. whilst lying in bed i could hear this buzzy hummy electrical sound. did my head right in, cos i could only hear it lying down in bed.... i would sit up, not there. get out and stand by my bed, not there. bend down by my bed and it was there again. put my ear to the floor and it was gone. no sound outside my room, nothing in the bedroom next door, nothing at the wall adjoining my neighbour the sound was purely at bed level and only in my bedroom, it so freaked me out for months. i thought aliens had put some sorta forcefield around my bed! (i lived alone so my imagination was able to run away). i bumped into my neighbour one morning and told him everyting i just explained above and asked him if he could hear it. he said yes, so we chatted about what it could be and it turned out he left his fishtank pump on all night and agreed to leave it off that very night and lo and behold.... no forcefield!! maybe your neighbour has a fishtank or fridge that sonic properties mean you hear it exactly at this time only?