Cause of Taco wate circulator noise?

I have hot water baseboard heat. When the water circulator is running (and the sound is not masked by the furnace burner noise), there is a noticeable humming / vibration that can be heard at the baseboard heaters. The noise is not from vibration within the baseboards. The noise seems to be coming from the Taco water circulator (model 007-F5) with the noise being transmitted through the heating piping. Note that this noise is not the noise one hears when there are air bubbles in the circulating water. My plans are to replace the circulator cartridge thinking that the cartridge bearings are going bad and that is the noise source.

However first I tried an experiment. After a heating cycle, when the furnace water temperature dropped below 100, I popped the boiler relief valve. A large puff of air escaped plus about two quarts of water. With the water intake valve open, I allowed the furnace pressure to build back up to its static pressure of 12psi. I then turned the furnace on. The noise was gone!!

Why did this experiment work? Am I missing something here?

Thank you.

I'm not a boilerman but will put forth a theory... perhaps the air was causing some cavitation in the pump.

No boiler guy either, but they should have an automatic air release valve. Your taco should have water lubed bearings meaning they last about forever.

Air in a boiler or baseboard is bad, very bad. Getting it out and keeping it out will increase comfort and system life while decreasing the fuel bill. Look the the near piping and air elimination devices.