Ok so the fan is not making the noise and you seem to be sure the propane detector is not making the noise. The Elixir converter used in my Jayco has the following feature: "If the power demand (lights, motors, battery charge) exceeds your converter amperage and the voltage drops below 12.5V, the audible alarm will sound. " You mentioned an old battery. If the converter is trying to charge a totally dead or bad battery, it could cause the condition indicated above. Simply disconnect the battery and see if the alarm goes away.