The medical term for this common Positive Airway Pressure (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 can occur more often with full-face masks.
Potential Solutions
Mask Adjustment: If you use a full-face mask, consider switching to a nasal mask or nasal pillow mask.
Chin Strap: For nasal mask users, wearing a CPAP chin strap may help reduce air swallowing.
Pressure & Device Settings: Other possible solutions include reducing the CPAP pressure setting or switching from a CPAP machine to an automatic positive airway pressure (APAP) device.
Important: Always consult your doctor before making changes to PAP therapy.
Article: Aerophagia: Symptoms, Causes, and Treatment