[pianotech] What's causing this vibration?

[Rebecca and John Silva]

Hi,

When playing a particular note on a piano I saw today (Baldwin L) a harsh vibration would sound. The vibration would only happen for a moment (1/4 second?) even while the note continued to sound, it was about 1/2 the volume of the note and only happened when playing hard. I worked with the owner for about an hour trying to figure it out and we were only able to determine it is based on pitch—if I detuned the note it would go away, and if I detuned the adjacent note to the offending pitch that new note would cause the vibration. 

I looked at everything I could, ensured every part/screw I could see was not loose, tapped on the bridge pins, and tried to figure out where the vibration was coming from.

Does anyone have any suggestions of where to look? Any common but elusive offenders? Two highly regarded technicians in the area were also apparently unable to figure it out.

Thanks!

John