When I tap on the left side of my head I feel hollow inside. I have no flu or suffered from sinusitis before. I just think that maybe a root canal on one of my molars is the reason because the tooth hurts sometimes even after two years of treatment. No dentist has agree to pull that tooth out saying that it looks healthy. May that be the reason? Or what else can be? I have no headache and I am not bothered it is just by tapping that I feel that. Thank in advance for your answer