I had this problem my whole life. I was literally known for blowing my nose.
Then I quit dairy for 30 days. It took me a couple of weeks to realize that my nose wasn't running anymore.
Now, if I eat dairy, I wake up with a runny nose and phlegm in my throat.
I don't know why, I have no idea how dairy consumption would affect a mucous membrane (maybe inflammation?) but it's worth a try?