My furnace has removable steel plate baffles. Sometimes as they change temperature while I'm loading, the blowers will cause enough vibration to make them rattle, it sounds like a moan I guess especially when the load door was closed.. If I tap on the baffle, the sound will stop until they change temperature again and I get the same sound. Its the steel creeping as it expands and contracts that sets up the right conditions for the baffles to rattle. At first I thought it was a blower going bad. I was enlightened to discover the rattling baffle.
Its a long shot, but check for something vibrating in your appliance.. Since, it seemed fine while you had it disassembled.