Hi folks, My heater goes through the normal startup process to the point where it starts blowing hot air. Then it makes sounds like gravel is bouncing around inside and it shuts down and throws the "Service Heater" light. I'm not an expert, but does this sound like a problem with the valve system that opens and lets water flow through the heater? Thanks for all attempts to "give a haircut over the phone". All thoughts are appreciated. Video of startup and sound: https://youtu.be/-6MIm9jbQy0