Sputum. For me getting this sputum?

Doctor from last 3 months I am getting sometimes light brown streak sputum, and yellow sputum when I wake up in the morning everyday. P.S I don't have cough but I have to clear my throat at regular intervals .I have Otherwise the whole day normal transparent sputum and sometimes it comes with light brown streaks. Doctor I have done 2 months back Ct chest with contrast , genexpert , sputum culture. But all were normal. My chest X-ray 2 months back was mild increase in brochovascular markings and mild hyper inflated lungs. I have tried allergic medicines too and also antibiotics but this sputum is still coming and everyday yellow sputum only once in the morning and sometimes brown streaked sputum during the day. Yesterday I have done a chest X-ray by my self and it is again showing mild increase in brochovascular markings and mild hyper inflated lungs. Doctor what could be the reason for me getting this sputum ? P.S I don't have any other symtoms .