Last Thursday I was back at the customer's house, two-plus hours away.
The low-frequency vibration seemed to be coming from many places, all
just under the soundboard, but I was unable to localize any.  Neither
tracking nor auto sustain were working.

Finally I located the stack cut-off valve, as John Grant had suggested.
Squeezing the vacuum supply tube caused the pitch frequency to increase
dramatically and get louder.  Disconnecting the output tube caused
the noise to stop.  I tried cleaning the bleed but that only fixed it
temporarily so I brought the valve back to my shop to rebuild.

My thanks to everyone who responded.  The shared knowledge of this list
has been extremely valuable!

Joseph Osborne - Antique Keyboard Restoration
Carlisle, Pennsylvania, USA