What is Cause of Whistling Noise When Circulator Shuts Off ?

I have noticed that often when the circulator shuts down a whistling noise is heard for a moment. Can someone comment on what is causing this to happen?
Recently, a baseboard radiator heater was replaced and I did attempt to bleed the series loop at the boiler. Could this whistle be an indication that there are air pockets remaining in the closed loop? Prior to the radiator replacement I do not recall ever hearing the whistling sound.
Thanks
de ~ Vince ~ Long Island in New York
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Vince wrote:

filler spouts with oil several times during the season. Maybe you can nurse it along 'til summer.
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Do you have a water auto feed valve, if so turn it off see if still occurs. May be a leak at the relplaced radiator.
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I have noticed that often when the circulator shuts down a whistling noise is heard for a moment. Can someone comment on what is causing this to happen?
Recently, a baseboard radiator heater was replaced and I did attempt to bleed the series loop at the boiler. Could this whistle be an indication that there are air pockets remaining in the closed loop? Prior to the radiator replacement I do not recall ever hearing the whistling sound.
Thanks
de ~ Vince ~ Long Island in New York
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So its not the auto fill. use your ear .
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