I just replaced my 5 yo heat exchanger because it was leaking. At the same time I replaced the bypass valve and the thermal regulator. Everything is operating normally except for this annoying whining sound that starts shortly after ignition and stops when the burner shuts off. If I run with the thermal regulator removed it is quiet. So I presume its a water flow issue involving the thermal regulator. I have verified that the bypass valve is installed correctly and not jammed. I have tested the thermal regulator. It starts opening at 120 degrees, but is not completely open till about 140 degrees. Is that correct, or should it be fully open at 120? Could the noise be because the regulator is not opening fully? Thanks, for any help.