Man I learn a lot here. What I found points to your humming as being a parasomnia. A para what people may ask? Well, to quote, "Parasomnia is a term used to describe disruptive sleep-related events. These behaviors and experiences generally occur during sleep, and in most cases are infrequent and mild. At times they may occur often enough or become so bothersome that medical attention is necessary".
To see where this definition came from and to read a concise and clear page on parasomnia's, go here:
This article discribes a vocalization on exhalation. I only read this abstract, but it was apparently considered benign and gave the authors a chance to propose another classification of parasomnias.
Vocalization during episodes of prolonged expiration: a parasomnia related to REM sleep
I also found this on vocalizations:
Being defeated is often a temporary condition. Giving up is what makes it permanent.
Marilyn Vos Savant
That which does not kill you makes you stronger-Friedrich Nietzsche
Friedrich must of had apnea.