I just replaced my 2 core stock rad to a 3 core aluminum rad, waterpump, harmonic balancer,thermostat, fuel pump and went from a 12 lb rad cap to a 13 lb rad cap. When I drive around for a few minutes and shut the car off I get a knocking noise from the cooling system. The upper rad hose actually moves a bit when it's knocking. The car is not overheating and I have heat in the car. If I leave the rad cap loose then it does not make the noise. Any idea what is causing this knocking noise and how do I get rid of it. I have a 1968 Mustang, 289, air, power brakes, power steering. I'm thinking maybe I should look through the garbage and put my 12 lb cap back on. The last thing is when it's knocking if I open the rad cap a bit then i can hear the a/f rushing through the top hose. My only other thought is my thermostat is faulty but I can't understand why I'm not overheating.