The medical term for this common PAP issue is aerophagia. Aerophagia occurs when the air delivered by your machine enters the esophagus and stomach and not the airway. It can occur when your machine pressure is set too high or if you are a mouth breather and are prone to swallowing air. Aerophagia occurs more often with full-face masks.
If you use a full-face mask, consider switching to a nasal mask or nasal pillow mask. For nasal mask users, wearing a CPAP chin strap may help reduce air swallowing. Other possible solutions include reducing the CPAP pressure setting or switching from a CPAP machine to an automatic positive airway pressure (APAP) device. Always consult your doctor before making changes to PAP therapy.