Piano Disc pedal mechanism problem

[John M. Formsma]

Piano Disc in a 1995 Young Chang G-150. (I'm assuming the PD system is 
about as old as the piano.)

When the system is playing the piano, the sustain pedal makes a knocking 
sound. It's more like wood-on-wood or wood-on-metal than a clicking 
metal-on-metal type sound. The pedal doesn't have this noise during 
normal human play.

I know little about PD systems except for the obvious things like 
reinstalling actions, basic electronics, etc. After 15 minutes of 
peering, replacing a worn felt, and head-scratching, I told the owner 
I'd ask others rather than learning with her $$.

As far as I can tell, the problem is not in the "actuator" that engages 
the pedal. (I did replace the felt under the actuator rod, so that can 
be ruled out as a source of noise.) The noise seems to come more from 
the rod that goes up from the PD pedal bracket into the action cavity. 
There is a rubber washer under the nut on the threaded rod that could be 
worn, but since I don't have those, I didn't attempt a replacement. 
Moving the PD pedal bracket slightly did lessen the noise, so I'm 
wondering if something is out of alignment. But nothing looked obvious.

Anyone know what this noise might be from, and how to tell a non-PD tech 
how to fix it?

Thanks,
John Formsma