Thanks Jim Katen for your insight. I have made an appt with a licensed professional to service the furnace.
This is not a DIY job by any means.
Even though tenants are responsible for such matters, and definitely not making excuses for the previous slobs who lived here before us but, we have a very negligent landlord who is hard to communicate with and doesn't care about their building (or furnace) one bit.
I always like to leave a place in better condition than when I found it. ☆