no such thing as a 'fan' on a water heater, only the electric element, which you would not hear, and the propane, which would fire like a jet...
maybe what you're hearing is a slight buzz of the electric element as it is being supplied 120v power...
'14 Palazzo 33.3 bunkhouse 34' diesel
KingTailgater2 Dish HD, 100w/5a SOLAR, BlueOx
110,000+mi since '14 - US, Alaska, Canada (also, '14 Gulfstream Amerilite and '07 ForestRiver Rockwood) : )b