I've had the same thing going on for a couple of months now.
I'm convinced it is some sort of blockage in me tubes, best thing to do I've found is drink plenty of fluids and always have some low backround noise.
This should help
http://www.rnid.org.uk/information_r...eets_leaflets/
Oh, and try not to let it bother you too much, just makes it worse.
