Why is it making a high pitched whistling sound?

I also experienced this. A sound from within the machine, a sort of wheezing sound that was apparent as I breathed in and out. It was a disturbing sound not having to do with an air leak from the hose, mask, or water tank, as I checked these possibilities first. I discovered the sound was coming from the motor itself. In my case, the motor in my machine was just five years old. I was able to replace the motor quite easily with the aid of a couple of YouTube instructional videos which were easy to find. I was able to purchase a replacement motor which I found on that large online shopping site. I believe it cost $129.00, and it arrived the very next day after I ordered it. I believe that just about anyone can replace their motor with the aid of an instructional video for which there are several on YouTube.