Nothing 
  Most ferrets don't make much noise.  This doesn't mean they're
  unhappy, it just means, well, they're quiet.

Clucking, "dooking," or chuckling
  Indicates happiness or excitement.  Often uttered while playing or
  exploring a new area.

Whimpering/whining
  Kits, especially, do this as a general excitement noise.  It can
  also be uttered by the loser in a wrestling match.

Hissing
  Frustration or anger.  Ferrets often hiss while they're fighting,
  even if it's just in play.

Screeching/loud chittering
  Extreme fright or pain.  This is your cue that it's time to go
  rescue your pet from whatever it's gotten itself into. It can also
  be a sign of anger.