Craig N3TPM has reminded me that there was a Canadian Noise Squelch module A24 and many have used it successfully.  I have the info on the module as well as the schematic posted. Circuit is nothing special - no IC's just standard components and  3 ea 2N2222's. Link posted and you will have to scroll down.
K4che

http://k4che.com/PRC77-Squelch/PRC77-Squelch.htm#other