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Whistling sound

I have a hot water oil fired system with 3 zones and a single circulator pump. Well over a year ago, I started hearing this whistling sound when one zone (main floor) was calling for heat. The sounds comes and goes at regular intervals of about 20-30 seconds. It sounds like a train whistle in the distance. Over time the noise seems to be getting louder. The noise occurs even if the oil burner is not running. I have tried to isolate the sound but cannot seem to pinpoint the source.



Since it is only happening with one zone, can I rule out the circulator?



Could it be the zone valve?



Or, could there be some sort of flow restriction within the boiler itself.
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