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? 
Sounds like something may be internally damaged and that frequency just happens to be the resonant frequency that makes the damaged part rattle.
What he said. What if you change the sound, be it with a software synth or the sound of the keyboard itself to something that may not be as heavy “sounding?” You still get the rattle then?
My keyboard has other synths that don’t make the rattle, but the Vibraphone one produces the same problem. I’ve contacted the vendor about a replacement so I guess I’ll hear back on Monday.