I had such bad post nasal drainage that I had to stop singing in choir. When I sing, I sound like I'm gargling through a vibrator, if you can imagine that. I saw an ENT doctor and even had a CAT scan of my sinuses, all normal. The ENT suggested turning the humidifer up, as did most people here.
For me, the solution was the opposite of most people. I found out by trial and error that I had to give up my humidifier completely. That has resolved 90% of the problem. Once in awhile it still bothers me. Then I use my Neti Pot.
Best of luck,