The spelling for that funnynoise you on youtube videos that microsoft sam makes. Also, master chief uses it in an episode of arby n' the chief made by digitalph33r.
Arbiter: What the hell is this on my back? I'm stupid?
Master Chief: Soi soi soi soi soi soi!
A phrase used by adding the term ROFL (rolling on floor laughing) and copter to create the term roflcopter. Soi soi soi is a sounds given to this "roflcopter" "machine"