Likely a little expense... But 250kV Tesla Coil would give you an unbelieviable effect. The very high frequency low current travels on the surface of conductors, so its shock safe. Makes lots of the sound you are looking for, but also people will see the little sparks and arcs on the chair and your victim, longer arcs to any thing metal or grounded. But if you aren't into high voltage stuff, might not be a good idea. But the point is, you can get Tesla Coil videos of the internet, and pull the audio track for use. Should be easier than trying to find the effect itself (many sites, with hundreds of sound clips to look through), where one Tesla coil sounds pretty much like any other.