My furnace is making a scratchy noise, almost like there is a metallic wire scratching the inside of the furnace. It is not a rattling sound and this is not its normal noise. It seems to be making it even when the heating or the domestic hot water systems are not calling for hot water. In other words, it seems like the noise is there even when the furnace is not actively working. I checked the furnace a few times and every time the noise was there. I also listened carefully to make sure that the noise was originating from the furnace and not from the hot-shot power vent, in case the fan blades were hitting the metal casing or something. I continue to get hot water, both domestic and heating, and have noticed no other changes. Please help diagnose this as the one plumber I called has not responded as yet and I am nervous that I might ruin the furnace by allowing it run as it is now.

Let it run till the plumber gets there. I had a bird that got into a flue at a customers one time. He actually lived for 2 days in the CO. I pulld out his dried out body once I found it.