Post Nasal Drip from CPAP or Thrush?

To all of you who have the sinus congestion problem since you started using CPAP,

I have had this for the entire 4 years I've used CPAP. It causes constant, severe post nasal drip, so bad that I had to quit choir, because when I sing, I sound like I am gargling through phlegm. My voice sometimes gets full of phlegm as well, which is very embarrassing over the telephone.

For the past two years, on and off, I've had periods where my tongue is coated in thick white liquid when I wake up, as if I put yogurt in my mouth and did not swallow it. It is pure white, is not lumpy or cheesy looking, and when scraped off, does not leave any sores, and does not usually come back until the next morning. It is nowhere but on my tongue, esp. in the back. There is none inside my cheeks, on my gums, etc.

My doctor assumed it was thrush, but extended use of several different thrush meds did not get rid of it. I am now wondering if it isn't just post nasal drip ending up on my tongue while I'm sleeping. It is the same color as my post nasal drip when I hock it up to see if it looks infected (sorry to be so gross). A sinus CT was normal, and the doctor told me it's not an infection.

Do any of you have this problem, and if so, what was your diagnosis and treatment?

Thank you in advance,
Klutzo