On Friday, January 15, 2016 at 2:47:38 PM UTC-6, wrote:
On Fri, 15 Jan 2016 09:48:27 -0700, Don Y
wrote:

On 1/15/2016 5:55 AM, Uncle Monster wrote:
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. ^_^

Not bearing/sleeve noise. Remove one door at a time and notice that
burner door removal causes noise to stop. Replace door and noise
returns (after a while). *Touch* (not "press"!) door in certain
places and noise again is silenced.

Door is rubbing on something.

Why it is rubbing now after 10+ years can only be because furnace is
VIBRATING more than it had in the past. Vibration suggests blower motor
(the only thing that moves and is massive enough) is no longer balanced.
This suggests a bushing/bearing is failing (blower itself can't magically
undergo changes in *mass*!)
Dirt buildup on the "squirrel cage" can put the blower out of
balance. Sometimes the dirt builds up evenly, then a chunk falls off -
particularly on furnaces run without a filter, or homes of smokers (or
both). Take the blower out and run it through the DIY carwash to
remove buildup. Don't knock the clip-on balance weights(if any) off) -
which happens when someone tries to scrape the crud off instead of
pressure washing.

I've had to remove all sorts of things from blowers and my brother had to crawl under a house to remove the plastic wrapper that came off a new filter because the lady of the house failed to turn off the thermostat before changing the filter. The plastic wrapper was sucked into the return air as she was getting ready to install the new filter into the return air grill. If it's been a very long time since you air handler blower has been removed and cleaned, you will have dirt buildup on the fins of the blower wheel despite having changed the filter on a regular basis. You can remove the squirrel cage from the motor shaft (scribe mark first), use a medium stiff brush to remove most of the dirt, taking care not to knock off the balance weight clips from the fins then put the blower wheel into the dishwasher to clean it and use the heat dry cycle to get it dry. Smear a little anti-seize compound on the motor shaft and setscrew before reinstalling the squirrel cage. Every few years, you should remove the blower and clean the whole thing including the housing plus vacuum any dust out of the cabinet. Do you have a straight AC or a heat pump? ^_^

Oh yea, I just remembered. Sometimes there are two setscrews in the blower wheel hub, one on top of the other to lock the first setscrew in and prevent the squirrel cage from coming loose on the motor shaft. ^_^

[8~{} Uncle Blower Monster