I'm stumped on this.
I could definitely understand a 'whine' on AC as the converter fan is quite high-pitched, but on DC, no way. Do I assume that you hear the noise with all appliances turned off on DC --- lights, heater, water pump?
There is nothing left except your 9-volt smoke detector in the ceiling (I have one in mine --- assuming yours has one too). I assume you're also aware that if you are powered up (DC or AC), and cut the power, the propane detector alarm will go off for about 10 seconds.
Otherwise, you have a hornets nest in there someplace...
I did have bees move into my fridge vent area and they buzzed all day. That was a fun time, until I evicted them.