Check out this video of Andrew and John Brinton of A-Pro Home Inspection Services checking out a furnace filter that should have been changed out months ago.
You can hear how noisy the system is as the furnace is struggling to get air in. This can cause the furnace to overheat, which can cause major damage and can be very costly to repair.
The best advice is to have your furnace serviced annually by a licensed HVAC technician, and then check the filter once a month to see if it needs to be changed.
And when buying a home in the Olympia, WA area, hire a licensed and experienced home inspector like Andrew and John Brinton of A-Pro Home Inspection Services.