This Sears model 139.53603, 10+ years old, makes a chattering noise when
going in the "door down" direction, even if the door is disengaged. (The
door is perfectly balanced; will stay at whatever position I release it.)

I've cleaned the track (using a carburetor cleaner), removed the chain
and soaked it in the same cleaner and used a toothbrush to aid cleaning.
In the control unit, I used the same cleaner to remove the old grease
off the plastic gears and relubed it with white lithium. I sprayed WD-40
on the high-speed parts, like the motor bearings and the shaft that
drives the gear that the chain wraps around.

There are no obvious defects in the plastic gears (like chips or missing
teeth). The only odd thing I notice is that the motor shaft moves back
(toward the gears, and away from the door) about 3/16" when the the door
is going down.

Thanks for your suggestions.

Ray