On Thursday, January 14, 2016 at 3:24:08 PM UTC-6, Don Y wrote:
Furnace has taken to "chirping" -- sound of two pieces of
metal rubbing against each other. Traced to one (?) of the
removable door panels (*probably* rubbing on the OTHER
door panel!).

As this panel covers the combustion area, flame is nearby.
Heat is to be expected!

My initial thought (when I thought it was the *blower*
area door) was to put some self-adhesive "felt" on one
of the offending surfaces (the "fuzzy" half of some velcro).
But, not too eager to try that with the presence of flame.

Any other suggestions?

I'll pull the blower out this weekend and check to see if it's
got a balance problem (bad bushing/bearing?) leading to any
possible increase in vibration (nothing obvious).

When sleeve bearings in a blower motor lose lubrication or the bearings start to fail, you will hear a chirping sound. You can some extra life out of the motor if you can get some lubricant into the bearings. I've drilled tiny holes into the sealed bearing housings in order to get oil in them. If you must replace the motor, spend some extra money, get a ball bearing motor and you'll never be troubled by it again. ^_^

[8~{} Uncle Motor Monster