Most likely you are swallowing air (aerophagia). It could also be a weak "sphincter" (I forget the correct name, LES? lower esophageal sphincter??) indicating reflux or GERD.
It is something you should discuss w/your sleep doctor. Maybe even bring it to the attention of your local DME supplier's RT who may then contact your sleep doctor. You may just need some expiration pressure relief or a lower set pressure or even a switch to an autoPAP. Or you may need evaluation by a gastroenterologist for reflux or GERD.
ResScan 3.10 - Resmed S8 ResLink & oximeter
ConTec CMS-50D+ oximeter - Philips EverFlo 5L Oxygen Concentrator
PR SystemOne BPAP Auto w/Bi-Flex & Humidifier - EncorePro 188.8.131.52