Question: What causes abnormal noises during heater operation?

Answer:

Slight abnormal sounds during heating are mostly normal phenomena:

Gurgling and humming noises are caused by water bubble movement and water pressure resonance inside the tank.
Faint electric current sounds are normal operating noise of internal electronic components.
Crackling sounds come from thermal expansion and contraction of the unit materials and relay closure.
Beeping sounds are usually triggered by voltage fluctuations, which can be fixed by powering off and restarting.

All above noises will not affect safe use.

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