One year, we had a wasps nest in the roofspace above the bathroom. When we were in there, we'd hear a low buzz like yours that seemed to move from one part of the room to another occasionally, but nothing to be greatly concerned about.
Until the first time dd switched her electric toothbrush on.
It must have given off the same frequency of vibrations that a wasp predator would, the whole room was filled with the deafening sound of what sounded like thousands of wasps, what a fright we got.
So if you're in the bedroom, be careful if you use anything that gives off any noise like an electric toothbrush 
But seriously, and it's much easier at this time of year as the days are shorter, look at the eaves or above the guttering to see if you can see a few insects hovering near your roof not long after sunrise and just before sunset, near where you think the noise is mostly, that should let you know if there's a nest. Then try and identify what type of insect it is, wasps are a pest, honey bees are endangered and will move on of their own accord usually. We've had bumble bees above 2 bedrooms and all you'd hear for a few weeks through the night in summer must have been just one who buzzed twice, sounded like zzzt zzzt every few minutes, but it stayed in the same place.
You may need to ring the Council and ask their Pest Control chap for advice, ours is very helpful.