I just replaced the radiator on my 99 Intrepid with a 2.7 Ltr. I let it idle for about 10-15 minute to circulate the new coolant and the engine started knocking loudly sort of under the intake between the heads. It is a factory rebuilt motor with about 50,000 miles on it with oil changed every 3,000 miles with Mobil 1. I was sort of thinking it was a emissions purge valve like the little Neons have that make that little clicking noise. I revved up the engine and the noise went away only to come back a minute or two later. This is the first time I have ever heard this noise and under normal driving conditions I have never heard it before. Any Ideas?