moisture in the cable will also create "noises" but will not show up on an SWR meter. I have seen it many times it doesn't take much. blocked condensation hole on an antenna will do it in very short order and "if" that migrates into the cable your only option is to replace them both. been there done that. if the receiver is tuned up wrong it will also create these noises. RSSI too low/squelch settings too low will have the same affect. have this w/Kenwood all the time. I have seen duplexers go bad and do this but that is pretty rare, unless it is poorly tuned. now if the transmitter is putting out junk this will wreak all sorts of havoc in you system and others miles away. rule of thumb is "if it doesn't sound right it probably isn't". as an example my 650 had a moisture problem last spring. it didn't sound right to me for a week or so, checked it w/Bird 43 wattmeter all looked fine. another week later it go worse. this time after putting the meter on it I went topside and opened everything up to inspect it. moisture in the connector and the cable. cut 6' off the cable to see if it was clean, it was. put new connector on it and changed the antenna. upon further inspection the fiberglass had become porous and that is how it got wet inside the cable, it migrated down internally. go figure.