I had horrible sinus headaches and post nasal drip. It bothered me so much when a doctor would prescribe me flonase, just treating the symptom. It was all food, try chicken and rice. I live off meat and dairy now, but dairy can give people mucus so I'd cut it out initially. Also with the meat, avoid deli meats and bacon in the initial tests as they have sodium nitrite, turned out to be a trigger for me. Gluten too. Look at all the FODMAPs. Carnivore diet is a good shotgun approach.