Tony - Good luck on the hiss, but I think the real problem is that your receiver just has a poor squelch gate. Anything that filters that out is going to make the desired audio sound like it's going through a sock. Only solution is either modify the rig or brew up an external squelch, which would be pretty easy... one quad op-amp and a handful of resistors and caps. Could tailor some nice audio, too. Sounds to me like you have two different receivers... what's the setup?