Well I'm not so sure it's a brain fart although I'm sure we have a few now and then both while dreaming and not.
I've read varied reasons why we sometimes hear loud noises (bells, clapping, buzzing etc) that jolt us awake and/or when we are just drifting off to sleep. There are the scientific theories that explain different functions of the brain with the how and why this may occur. I've also read that noises like bells, pops, bangs and even buzzing are related to astral travel. Another explaination discussed how it may relate to our spirit guides and the ways they get our attention in effort to communicate with us.
Astral travel was the version that spoke more of the louder type noises and it seems naps are fairly common times when ppl travel. Do you recall dreaming prior to this happening and if so what type of dreams are they?
I often get a buzzing in my ear as I'm drifting off and have been jolted awake at times (mainly when just dozing off) by a loud noise. Sometimes it's hard to tell if that noise was just in my mind or if it was coming from in the room. =)