Weird post nasal drip

Hello to everyone. New here. About 12 weeks ago I developed a heavy chest with a post nasal drip cough. I coughed up clear mucous quite a bit. This has cleared away but the post nasal drip cough continues and it is driving me crazy. What is unusual about it is that my nose is relatively clear and not blocked and yet from time to time I can feel mucous in my throat and that awful tickle that makes one cough. It appears worse when I lie down, especially if I roll onto my left side which causes a bit of air to push up and the ticklish feeling starts well down in the throat before getting near the Adam's apple area.

Most of the day I am now okay but every hour or so there's a build up and I get the ticklish cough. Sometimes when I cough I get a bit of clear to white mucous firing out onto the tissue paper. I have had a chest x-ray which showed no cancer in the lungs and that the lungs are relatively clear BUT that the bronchi were showing signs of collapse (which the doctor explained to me but I didn't quite understand). I was due for another x-ray next week but my doctor wants to look closely at my thyroid gland so he has order a CT scan instead, including the chest.

My GP (MD...I'm Australian and we call MDs GPs) tells me that it is entirely possible to have post nasal drip cough even if the nose is relatively clear. I know it can be caused by gastric issues (GERD) but I take Tasac and Nexium and there is no burning at all and the situation there is well controlled.

I have used saline sprays and Dymista but this has not proven effective. Is there anything else I should do? Is my GP right in saying that my nose might be relatively clear but that all the mucous is running directly down my throat? Thanking everyone in anticipation.