There are a number of possible causes, but the first thing to try aside from checking mask fit for over-tightening and (less likely) looseness, is to make sure your EPAP = 4 (some machines allow 3, even better) unless you have a pre-existing obstruction or other lung condition. Some PALS are needlessly in the AVAPS-AE mode or have an EPAP set too high.
If EPAP is appropriately low, the next thing to consider is Vt. If your lungs can't handle the volume they're getting, exhalation can be where that first shows up. It's sort of like loving cake while you're eating it, but then comes a sugar crash. This hypothesis can be tested by reducing Vt by 10%. Most PALS' are set too high at machine delivery.
If those are not the answer, you can try adjusting trigger sensitivity (if your machine is on AutoTrak, you will need to change trigger type to Flow Trigger first). If sensitivity is too high (expressed as a lower machine setting), you may not get enough time to exhale. If it's too low, air could be getting trapped. Cycle sensitivity (how soon the machine switches to exhalation pressure) can play in as well, along with rise time (how fast the machine ramps up pressure in inhalation, not to be confused with AVAPS rate, how fast the pressure adjusts to the amount of air you are moving vs. your target).
If not related to machine settings/mask fit, vocalizations can be considered a parasomnia and they may increase with fatigue in the experience of other PALS, so that would be another thing to try to manage.
Of course, I'm always happy to look at anyone's settings in the context of whatever you're experiencing, apart from vocalization.
Best,
Laurie