I have found it useful to turn off the automatic squelch, then turn on all your equipment, one at a time. The squelch background noise will rise with any offending unit. The Cambridge model-20 and ILEC SN-10 both add significant background noise, so do some of the power supplies for additional units.. Remember the radio is doing exactly what it was designed to do, It thinks a weak signal is bing received and sends that 'weak signal' to the speaker. I have found that moving offending equipment away from the radio and its antenna have solved the problem. Have patience, you'll need it!
JJ