I think you may be air sipping rather than full mouth breathing. Personally I didn't know that was even possible until a few weeks ago when I woke up slowly, twice, hearing a sound I could not identify. As I became more awake I thought to check that my lips were closed and was surprised to find they were slightly open. When I closed my lips, the sound stopped. Both times my mouth was somewhat dry but not as much as it used to be when I had a habit of occasional full mouth breathing. The only possible cause I identified of the air sipping experiences was that they occurred when I had minor cold symptoms.
