|Heating with large noise/ sound||Dear Sir/Ma'am, heating sound is a normal pysical phenomenon, because bubble will be expansion when temperature increase, an old saying like"boiling water no sound, and sound water is under boiling" ,so don't worry, That is ok||
Dear Sir/Ma'am, this is low pressure problem. If pressure of water supply is low less than 1.5 bar(5 floors' pressure ), the solubility of air is low, so bubble escapt to water when heating, that is making sound.
I suggest that you may close inlet valve and outlet valve, power on water heater and keep it on heatng, 5 minutes sound will come down
If still can not solve your problem, please contact with service center.