why do i hear a clicking noise in top radiator hose

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my truck is running hot at lower rpm. when i accelerate it gets colder and will run on thermostat temp until it shifts to lower rpm. i hear a clincking pulsing noise in top radiator hose almost as if it is a pressure build up of some sort. brand new radiator ruptured but was under warranty so i put new one in. new water pump also.