My six-month-old ferret, Brae, also makes a hissing noise.  I can't figure
out if he does it in anger or excitement.  He often makes the noise when
he's playing with the cats, and he's done it when he's gotten spooked.  He
also did it the other day when I was scolding him about getting into
something (almost like he was scolding me for spoiling his fun!!)
[Posted in FML issue 2283]