Update - I began to suspect that the thermostat was only needing to open a little since I have a 3 core desert radiator with a 6 blade fan (lots of cold water). So I placed some cardboard in front of the radiator for a few minutes and the hammering at the thermostat went away within about 30 seconds. This is the third thermostat so I know its not defective. I am switching to a clutch fan so that it will pull less air until the water temp comes up. This should cause the thermostat to open further. This issue took me a year to solve. Hope this helps anyone else in the same predicament.