Noisy mask operation

Have been on one mask style or another for 10 years. Starting using the Simplus full face mask about 8 months ago. I use it with a humidified system (per doctor). 2 - 4 times a week, gurgling, whistling and/or a "slapping" noises wake me up. I can stop the whistling by brushing a finger across the array of small holes on the front of the mask but the other noises persist, especially the "slapping" noise which feels like the valve at the front of the mask is the culprit. I have replaced the mask once but these phenomenon continued. I really like the mask but these noises are infuriating! Any suggestions?

Sounds like maybe a little moisture in the mask.
Maybe a little condensation of the moisture in your exhaled breath mixed with the moisture from the humidifier (assuming you are using one).
We call it rain out.

Exactly which machine and model are you using? Any chance you are using a heated hose?

I'm using a REMstar Series M machine with standard, unheated hose

If what you are experiencing is related to moisture/condensation in the mask you could try several things.

Hoze cozy to try to keep the air in the hose warm enough so that when it gets to the mask it is warmer and won't cool to the point of releasing the moisture.

Reducing humidity level coming from the humidifier in hopes the moisture level in the mask stays below saturation for whatever air temp is in the mask but this may not offer much help...rain out in the mask can happen even to people who don't even use a humidifier if it is the moisture in the exhaled breath that is the trigger.

Warm your bedroom air temp a bit to see if that helps keep the air from releasing the moisture.

Try routing the hose and machine so that everything is lower than the mask in hopes of gravity helping the moisture to go back down the hose to the humidifier.

All this is assuming it is moisture that is the culprit. It takes very little moisture to make a lot of ugly noise.
Why this mask and not another mask? I don't know unless the thinness of the silicone just allows for that air to cool a little more than another mask might.

Unfortunately the M series machines don't have the heated hose option but stand alone heated hoses can be purchased.

One other idea and it's a bit out there....I have known of people putting a little bit of some sort of absorbent material in an out of the way corner of the mask to help absorb moisture.

Oh..one other thought...mask liner in hopes that the liner might absorb some of the moisture.

Again, all this assuming it is moisture that is the culprit and it's highly possible that moisture is the issue from what you are describing.