The furnace was making a bit of a growling noise momentarily when it kicked on. Now it's a steady rumble with a smell of burning rubber. It still produces heat, but we turned it off for now. It seems to only do it when the fan is on because turning the fan off and leaving the unit going makes the noise stop. I assume noise and odor are bad, but does anyone have an idea of how bad? New blower, or new furnace? $300 or $3,000?