All masks have exhalation ports (look for a tiny cluster of holes) that allow the escape of carbon dioxide. The higher the machine pressure setting, the harsher this escape flow will be. Some masks have better air diffusion features than others.
When routinely cleaning your mask parts, these tiny holes must be checked to make sure they are not soiled and clogged by body oil or bedding lint. When clogged, they can cause the mask to make a whistling noise. Use a sewing needle or toothpick to keep the holes clear.