Odd churning sound from furnace

[BugsyJ]

Just parked our Airstream after a week-long trip in which we used our furnace only once for about 10 minutes. Shortly after unhitching while cleaning it out the furnace started making a weird churning sound, almost like a car trying to turn itself over.

There are two ways to get this churning noise to stop:
1) Turn on the furnace - the churning goes away and it seems to function like normal.
2) Put the trailer in "store" mode, which is what we have it in now.

Hoping this is an easy one for some of you more mechanically-adept folks. Ideas?

Thanks in advance...