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Posted by Frodo on December 19, 2004 at 20:35:24:
At the end of an extended heat cycle, such as in the morning when the automatic thermostat raises the temperature, the system hums. There is no hammering, just a loud, low pitched hum that goes for maybe 30-60 seconds. When it happens the the burner is off and there is over 2 psi on the gauge. Hard to tell where its coming from but I can feel the water feed line vibrate. It is a National-US Model 4-37 installed in 1958 and otherwise does an excellent job of heating this old house. Any ideas? Thanks!