If it is running from the far side propane tank, try switching the regulator to the near side tank. Sometimes there is problem getting sufficient gas from the far tank to run the furnace.
Also try this. Shut off both tanks, and burn off any remaining gas at the stove top, and then shut off the burner. Then very slowly crack open the tank valves, if it is quiet, you should be able to hear the gas flowing out of the tank, let it flow slowly until you can no longer hear any flow, then open all the way.
If either of these tricks fixes the problem, let us know, and I'll try to explain what might be happening.