Humming electric panel?
I've seen/heard this happen when a breaker is very close to tripping. That is close to the maximum amperage for that breaker being used.
Also they will "buzz".
If you have the installation instructions for the water heater, see what size circuit they recommend for that model. Or find the manual on the internet at the manufacturer's web site or call them.
Also a heating element for the water heater may have been replaced with a higher wattage heating element. Check amperage draw for that circuit with an amp meter.
And home water heaters with dual elements are designed for ONLY one heating element to be on at a time. If all cold water, the top element goes on first, then when the water reaches the set temperature, it turns off and then allows power to pass to the lower heating element. That runs until the temperature in the lower part of the tank reaches the set temperature.
Commercial water heaters will run both elements at the same time. So might want to check that only one element is on at a time (or have that checked).