What Causes Persistent Sinus Headaches?

I have experienced headaches for years, focused in the sinus area behind the cheekbones and eyebrows. ENT doctors have said there is no infection. I have no mucus discharge. However, associated with the headache pain is a feeling of shifting between the sinuses, especially when lying down, and a sinus popping or clicking noise. Sometimes the sinus shift eases the headache pain.