Before I changed the elements and thermostats on my water heater, there was a slight hissing sound. Now I don't hear any sound at all. One plumber told me I should still hear something on the top element when things are working properly, another said it is better that you don't hear anything. I tested the voltage to the elements and everything is ok. I checked the ohms and all is well. My question is, is it possible that the water heater is working but doesn't make any noise at all?