thumping from radiator during coolant purge is very common I am a mechanic in connecticut and I have seen this often it is usually caused by not enough water in the engine at start-up. Normally if I put something like this together I try to get the cooling system together early and then I fill the radiator with coolant do something else like hook up cables or throttle and then fill up the radiator again. I generally fill the radiator 6-10 times until it takes no more coolant. Then when I start the engine it has already gotten all the air out. pressure from under the valve covers is copletely another story, Pull the pcv valve from the valve cover put your thumb over the hole take off the oil cap put your hand over that hole if you feel a pressure build-up you probably have blowby past the rings or you have compression leaking through a head gasket to an oil passage. with the pcv installed you should have a slight vacuum.
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