Persistant cough

Mrs Jayalakshmi Venkateswaran  ( E Mail : ***@****)
Age : 70 years
Ht : 160 Cms
Wt 76 kgs
                                                        Medication

HPERTENSION( 27 years)
1. TOZAR 50 (LOSARTAN POTASSIUM)                                     0-0-1
2. Natrilix SR 1.5  (Indapamide Sustained Release)                      1-0-0

DIABETES(17 years)
1.Human Mixstard ( 30/70 Insulin )                                          22-0-10 ( Units)
2.GLYCIPHAGE 500 (METFORMIN)                                         1-0-1
3.Gliclazide (DM 80 )                                                               1-0-1
4.Acarbose 25mg ( Glucobay)                                                  1-0-0

BRONCHIAL ASTHMA(11 YEARS)
1. SERFLOW ROTOCAPS 250
( Salmeterol & Fluticasone Propionate)                                      1-0-1
2. TIOVA ROTOCAPS(Tiotropium Bromide)18Mcg                       0-0-1

THYROID (5 years)
Thyrox 50mcg 1-0-0
(ThyroxinSodium)

PROBLEM
Productive Cough  for over 12 months with Thick sticky Green discharge in small quantity even though on steam inhalation on a daily basis.
Earlier cough used to be there off & on for over 3 to 4 years
More after use of Rotocaps & rinsing with water.
Cough pronounced immediately on lying down at bedtime in the night for
about 20 mts and occasionally during sleep.
The flem (phlegm) seems to be continuously getting generated inside the system.
A swelling around Lt Knee for over 3 years. Not painful but on exertion
pains.
Treatment about 3 years ago by ENT suspecting GERD? not effective.
However never had dry cough or symptoms of reflux.
Investigation results in the last fortnight.
BP & Blood Sugar under control.
Chest X Ray OK
No Weight loss. Normal appetite. No blood in sputum.
Thyroid Function tests T3,T4 & TSH.  Urea, Creatinine, & Lipid Profile OK.
TLC/DLC – Normal. Absolute Esnophilia Count (AEC) -450
CT Scan chest – Normal. Chest Physician Confirmed Lungs clear
Echo – No major abnormality
I daily walk for 30 mts slowly without any problems.


Please advise
what are the possible causes for this cough?
Are there any further investigations required for identifying the underlying  causes & eradication of this Problem?
Even after visiting specialists such as a Chest Physician, Cardiologist, & a General physician the cough remains. Kindly Advice