there is only one way to fix this aside from opening a window a tiny bit in your car, that is to get one of those plastic sunroof air deflectors that mount right in front of your sunroof. The reason this happens is because there is air being forced into the sunroof by the movement of your car through the air, but since your windows are up, the air inside is not easily replaced by the air outside that is trying to get in. It is the same principal as when you are pouring something out of a jug and it glugs out in big spurts instead of flowing steadily. The material inside is trying to get out, and the air outside is trying to get in, both cannot pass through at the same time because you are forcing all of it to the neck of the jug, so they kind of take turns where all of one goes out, then all of one goes in. The sound and sensation you feel when this happens is the air in your car constantly increasing in pressure and then decreasing as air escapes. So it all boils down to this. Easy way, open your window a little bit. Hard way: Buy a sunroof deflector.
