Why is my electric water heater making an intermittent groaning noise?
Groaning, rumbling, or other low-pitched noises from a hot water heater are typical symptoms of excessive sediment buildup in the heater; normally, flushing it via the bottom drain valve is recommended to clean out the sediment buildup.  If not cleaned out, eventually the heater's lower element will burn out due to being buried in insulating sediment (calcium carbonate).