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