Last July I noticed a loud purring noise coming from the outside wall - like a cat purring but louder, slower and deeper in tone. My initial thought was that it could be a cat sitting outside on the deep window ledge and that the sound was somehow being amplified. This noise went on for several nights (I didn't notice it during the day) and I started to get quite concerned. It was unclear exactly where on the wall the noise was coming from and I tried banging various sections of the outside wall with my knuckles to see if "whatever it was" would react - however the noise just went on.
Then one morning I suddenly woke with a half-memory of reading, or seeing on TV, something about bees or wasps beating their wings to ventilate their nest. This prompted me to check outside and sure enough wasps were entering and leaving the thatch by the bedroom window every few seconds.
I phones a local exterminator who agreed to come round that afternoon. My wife thought she's do a quick clean and tidy of the bedroom before he arrived. Fortunately I was standing by the bedroom window with a ring binder in my hand (I was just about to do some filing). As my wife passed the duster over the plasterboard area above the window a section of plaster fell into the room revealing the wasp nest and countless wasps. A few wasps got into the bedroom before I managed to slap the ring binder over the hole (matter of seconds only). My wife got stung three times before she managed to squish them.
The wasps behind my ring binder were buzzing VERY angrily (I could feel the vibration) and I had no choice but to stand there holding the binder over the hole while my wife rang the exterminator again with a panic request to come round immediately. I stood there for 40 minutes before he got to us and the wasps never stopped buzzing. He dealt with them and we had a chance to see just what had happened.
The nest was huge - completely filling the meter square cavity between the plaster and the thatch. The wasps had eaten (removed) a 6X8 inch oval of the plasterboard and just left the egg-shell thin skim of plaster.
This incredibly fragile piece of plaster could have broken through at any time (when we were asleep in bed ) and the room would have filled with wasps. If I'd have banged that section of wall with my knuckles the previous night....it was a VERY narrow escape!
If I'd not been there at the moment it collapsed with something to cover the hole it could have been disastrous!
Several days later I removed the wasp nest and seemingly hundreds of dead wasps. So I'm very grateful to the loud purring sound that alerted me to the fact that something was amiss!
I have no idea but got shivers just reading your post!!
OMG ACK!!! One of my very worst nightmares!! 
