AF 1150 question ?

Hey campers: Tonight my wife asked me why our campers water heater was making a odd noise. It seemed to be happening only when the hot water tank was heating by the electric 110 v circuit. It sounded like a air leak sucking or blowing air. After turning the water off you could hear the noise inside and out but it was coming from the outside water heater tank area. I thought it might be a the pressure relief valve but only on the electric setting. ? seemed odd, Thanks for your help, be safe.