I wish that was an option
I have like a 1-2 inch gap behind the furnace in a very tight area where the entry is, so no way to reach it from inside without removing the furnace. I've tried to reach it from under the house, but that was a lost cause for me. I need someone really, (really) small to even attempt to get to the area, as my crawl space is extremely small and a total obstacle mess. I don't know how they put the duct work in under the house. I guess with enough cash someone would be brave enough to do it
---I think you may have given me an idea with the tin foil. If I can't get to the source, maybe I can close up the sides behind the furnace. Hmmmm, at least block the next entry point into the house.