legacycentral bbs

I really honestly don't know whats creating this noise, but it only does it on heat with it on the hot position, on the cold side, or vent or other settings it doesnt not do it, the heat side on the hottest temperature does it after the car gets warm inside, and you have driven for a few minutes. Its a really loud banging noise, and its not a steady noise im hearings, its like knock........knock.knock........knock stop for a few seconds, knock..........gone, then comes back. I do have a missing fin in one off the vents. If it fell in can it be in the fan possibly, and its hitting it? Also i don't think this has anything to do with this, but i did change the thermostat to a non-oem thermostat, and i installed a new gasket, and on cold days it warms up with no problems. And reused the same anti freeze. What else can be causing this noise, its so frustrating when you drive around now, because it does it all the time!!!!!!