Our gas furnace has been loud for a while, and getting louder. We got a furnace check-up this fall and the technician said, that noise is your inducer motor. They fail often on these furnaces and your furnace is old. Sounds like yours is on its way out.
I started researching electric replacements for gas furnaces, i.e., heat pumps. That picked up in early January, when my friend George sent me the hot-off-the-press guide from Rewiring America, Electrify Everything In Your Home.
The day after I started reading it, I woke up to a chilly home. The inducer motor had failed.