I hadn't considered that because it doesn't seem like I'm fully exhaling through my mouth. HOWEVER, in the absence of any other explanation, I will try your suggestion.

It's nearly 0230 and I have gotten out of bed because this problem is driving me up the wall and I finally have done a web search. My BIPAP and mask (Swift FX) work fine; there is no humidifier. But ~~I~~ am making noise via the mouth. Sometimes it's a whistling sound as air travels between my teeth...