Atwood water heater humming noise

I had the same problem last summer. While having a shower I could hear the noise above the water flow and wondered what the hey that noise was. I discovered it was coming from the heater. When turned off the noise went away. I emptied the heater and refilled making sure that I opened the pressure relief valve to empty out the air in the tank. It didn't make the noise after that. The only thing was that we had been on the road for almost 2 months at that time, using campground electric, so I cannot figure out why it happened. Just brought my rig home yesterday, filled up with water and have the heater on electric. When I get outside I will be checking further. By the way I did get on line and googled my problem at the time and it was recommended to drain the tank, refill and make sure all the air was purged from the tank. Good luck