Opel, I think you should take Phreak to your vet and have him/her look into
the possibility of cardiomyopathy.  Ferrets with this disease generally have
enlarged hearts (causing the coughing) which do not work as efficiently as
normal hearts.  If this has been occuring for a while, I'd consider this a
possibility.
 
Cardio can be controlled by meds.  I have a 5.5 y.o with it who was
diagnosed last October, was given 4-6 months, and *now* may have as much as
a year ahead of her.  I believe there's a medical FAQ on this as well.
 
Amy & Dave (and their gaggle of giggling ferrets)
RIP Ian and Elektra
[Posted in FML issue 1639]