With age, the electric heating element will get a coating of mineral deposits on it or debris from the anode and mineral deposits can build up in the bottom of the tank, insulating the element from the water. When this happens water will boil on the heating element. This is not like boiling water in a kettle, but very small bubbles that are reabsorbed into the water. I think the sound is like a sizzling sound, I find sounds can be hard to describe. While our sounds are not described the same, this is the only thing that I know of that would cause a sound while the electric element was on. I have heard it from my 2002's water heater, before I replaced the anode and cleaned out the tank and from many portable spas with electric heating.
2007 Limited SE 40fdts
Las Vegas, NV