My wife called me on the road to ask me if I had left any "machinery" running in the trailer. I thought for a moment but came up blank. I haven't been in there since we unloaded from our trip a couple weeks ago.
She said it sounded like drilling or grinding every few minutes, and she didn't hear anything this morning.
I asked her to check back and see if MAYBE someone had left the pump switch in the on position. It turns out that this was the cause. You see, I always empty the majority of water in the tank before we break camp, and I must have left the switch in the on position. Cold weather and some time must have got things to the point where the pump was cycling on and off at random intervals, scaring the daylights out of everyone.