Whenever the water is reheated after dispensing hot water or about every two hours, the heater actually boils the water and makes noise. This is normal as it is designed; but it could have been designed to reheat water more slowly to avoid boiling just by using a heating element with higher electrical resistance. Another design technique would be to use a heating element with a larger surface area in direct contact with water. It is the temperature of that contact surface that determines whether boiling will occur and the rate of boiling.
However, as is, the noise could be mitigated by using a rubber mount. That would best be done by building in a rubber hanger on the unit; another design change.
A mechanical engineer