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Opp's, time to rebuild the RF board. Could be the Command board too, could be the Rec frontend, 'too'. Just switching out boards can get you in trouble, as a problem on one can make the other one useless.
The noise goes from the RF board up thru the Audio IC on the Command board where the noise gain is set by the microprocessor, like a 'softpot', then returns to the RF board for the accual squelch detector circuit.
Sounds like not enough noise or lack of noise gain in the Command board.
I would like to hear what you find, I have the exact same problem on one of my Spectra's, it can be a pain when scanning as the radio unsquelches very easily. Was wondering if as these radios age they will start to exibit common issues like this.