Does anyone have problem with post-nasal-drips in the wind pipe?

Most of the mucus from my post nasal drips goes down to my wind pipe forcing me the need to cough it up. Only a very small amount goes into my throat. I am really suffering from the continuous coughing and nose blowing. Yet all types of nasal sprays have not worked. Allergy tests are all negative and CT scan of my nasal passage shows nothing.
Any suggestions?