Loud, high-pitched whistle from nasal mask vent

I am a 10-year + veteran of using a CPAP machine, and 5-year + veteran of using the AirSense 10 model and the DreamWear nasal mask, which I clean and replace as recommended. Both have been awesome until very recently. Now most nights, but not every night, the elbow vent makes a loud, high-pitched whistle sound that I am at my wit's end to stop. There is no water droplet getting in the way, no condensation, no object obstructing airflow. The ambient air temperature is higher than 75F. Changing between older and newer masks makes no difference. It's impossible to fall asleep, let alone stay asleep. I don't know what else to try.

I do not have experience with that particular mask. However, it sounds like you have switched masks with no change to the sound. It would seem possible the sound is coming from somewhere else other than the mask. A leak in the hose? From the A10 machine? Have you changed the inlet filter in the machine lately? Are the seals between the water reservoir and the machine in good condition? I have found that the water reservoir has to be firmly pushed into the machine to make it seal. Do you have an older machine to try instead of the A10?

Thank you for your prompt response, Sierra. Indeed, after writing my question, it occurred to me to try my "old" machine. It was one of the Phillips Respironics models affected by a massive recall a few years back. Luckily, I registered as an affected user and, a few weeks ago, was able to exchange my old machine for a brand-new DreamStation 2. I cleaned all the parts, tested the machine, found it adequate, re-cleaned everything, and put it away. It was ready to use last night and it did the trick: no annoying noise. I will now use this knowledge to troubleshoot what is wrong with the A10. The seals look good but warrant further investigation. Many thanks again!