You have my sympathy.
I have exactly the same symptoms as you describe.
They began 12 months ago after surgery for gallbladder removal. At first I was told it was a cough from the anaesthetic.
Chest xray, CT scan and blood, allergey & sputum tests all negative. A bronchoscopy showed mucus in my lungs but biopsy was normal. Ive had steroid & antibiotic & various asthma medication treatment but nothing stops the terrible productive cough. Its worse at night and first thing in the morning but with me all day.
I also have developed chronic sinus trouble & am waiting for the results of a CT scan on my sinuses.
I have recently been in hospital for treatment with inhaled lignocaine again no improvement- worse if anything. My respiratory consultant has declared defeat and passed me to an ENT specialist.
Now I am told that its more than likely GERD.
I am being treated with Omeprazole 40mg daily by my G P which he hopes might be the answer if given along with treatment for post nasal drip and a sinus wash out!!
I cant take benadryl or similar decongestants/cough medicines as they react with my Parkinson's medication so Ive not triedthe 'magic mouthwash '
I shall be interested to hear how you progress and if you find any relief from the cough.
meanwhile we must keep positive - someone somewhere must have the answer to our problem. If you find them before me please let me know