This past November I had the gas company come out to perform maintenance (cleaning, etc) on the furnace and shortly thereafter whenever the furnace would run there was a rattling/vibration sound coming from the vent in the wall/ceiling in the room next to the furnace. Had the gas company back out to look at it but they couldn't figure it out. Then a couple months later it stopped.

I assumed it was some loose debris that due to the improved airflow from the cleaned up furnace was being rattled around, however, now we just turned the A/C on for the first time and once again the rattling noise has started back up?

Does anyone have any ideas as to what this might be?