I've been having a heck of a time with my cpap, and waking up with a belly full of air every night. Pretty painful! They told me I probably needed a chin strap as it sounded like I was "swallowing" air. I went in and they printed out my sleep history for the past two weeks. I am not opening my mouth, so a chin strap isn't needed. Instead they think I may be sleeping with my head tilted too far back, or sleeping with my head tilted too far forward with my chin tucked towards my chest. Both positions will allow air to enter your esophagus and stomach. So last night I made sure to sleep with my head and neck aligned with my spine and I didn't have any air in my belly overnight! Hooray! I hope this is the answer I needed. Just thought I'd share in case anyone else is having this problem.