Coughing all night

Hi mamas, my girl gets sick every couple of months but the cough lasts forever. I swear she’s had a cough for the last eight months. During the day it’s not too bad but she coughs all night. Doctor has checked her lungs and they are clear so I think it’s post nasal drip? We have a humidifier in her room, we elevate the head of her mattress, we give honey and homeopathic stuff, Vicks, but nothing works. The sound of her cough keeps my hubby up all night while I’m with our new baby. Any tips?