The niose is called insipient Boiling, it is caused by the disolved oxygen in the water which is forced to expand (boil) with contact on the Heating element. This forms a Oxygen bubble,
some of which collapse some stay in suspension. As the Oxygen in the system is reduced by air vents the noise reduces. This usally takes three or four days. It will reapear if the the system water is drained and replaced with new water.
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