Re: Why do I sometimes hear guitar sounds in my head?
Hi, I agree with Em above, that it could be tinnitus.
I also have tinnitus, usually of the type described by Em - high pitched hissing or whining, sometimes a low drone... but once I woke in the night to a different type, much like you describe - a vague musical sound, almost like a repeating chorus, much like it was on a radio in a nearby flat or something.
Luckily, I'd already read that this is just another type of tinnitus (albeit a curious one!) and so I wasn't freaked out as much as I would otherwise have been.
Take a look here for more information: http://www.tinnitus.org.uk/musical-hallucination
As ever, the main advice would be - speak to your doctor if you're concerned.
__________________
"What you resist, persists." - C. G. Jung