Reply to Strange speaker rattle/distortion

When playing the note A5 on my keyboard (and also the nearby notes) I get an odd rattle sound from one speaker (Wharfedale 9.1). It only happens if the volume is at medium level or higher, and only happens on this particular speaker.

Everything else in the chain has been eliminated. Wires have been switched, playback set to Mono, grille removed etc.

Any ideas?