I also have a Warwick fretless. It should not be hissy unless you are favoring one of the pickups from the balance knob. Make sure it is centered and watch out for lighting fixtures. They tend to add noise.
Human Base x.oc5, Warwick Corvette 5 Fretless, Fender Amer. Jazz 5, Squier Infinity 5 (x2).
Buying used strings is like buying used underwear.