Water Heater--Noise?

[barmar]

Just noticed that our water heater is making a strange/different noise--haven't heard it before! Only makes the noise when on electric and it's a low, hissing noise almost like water running but no evidence of any leaks. Can hear it inside the rig/kitchen area and certainly at the water heater outside. Any ideas? Should I turn the electric off and just use propane until I find out what's going on? Thanks, B