High pitched noise from machine

My husband wears the Cpap most of the day and especially at night. For the most part there is no noise. However, once he falls asleep, you can hear the air going in and out and then the worst thing is that there is a high pitched whistle that he doesn't hear, but I can. It keeps me up all night. Gives me a head ache. What do you do?

The first thing to do is find out where the whistle is coming from.
Is it coming from the machine or from the mask, tubing, etc.?

Since it's happening when he's asleep, it sounds like a leak or maybe the vent flow striking something causing the sound.
It would also help if you'd tell us which machine and mask he's using.