I believe that the only Whistler models that this would apply to would be the WS-1095/1098 and TRX-2. The 1080/1088 and I'm pretty sure the TRX-1 has an old-fashioned potentiometer-controlled squelch that's only adjustable via the knob, and it doesn't use level numbers. Set it at about 10:00 and if you still aren't hearing something, gradually turn counter-clockwise until the radio hisses, then turn it clockwise until the hissing just stops.