I have a freshly rebuilt 351C, car is a 1971 Torino GT. It starts and runs perfectly. I let it run for about fifteen minutes while adjusting carb, checking timing. When I turn it off it keeps making a popping sound right at the thermostat, like its opening and clossing hard,(and fast, but not regular interval repetition) I replaced thermostat thinking that the parts store sold me a bad one, does the same thing. Swapped the waterpump also. No difference. I ran it without thermostat to make sure that it was pumping water, and it is, Also have a brand new aluminum radiator. The thing that bothers me is that the noise sounds mechanical, but doesn’t start until the water heats up. There's no EGR reactor valve to cause such a problem. All parts in engine replaced brand new. None of this happened prior to rebuilding, what could be wrong? Leak somewhere that I can't find? Vacuum problem somewhere? Sratching my head here. I would really appreciate any direction you guys could offer. Thanks !