Did you ever resolve this issue? I just bought a MXT575 from e-bay and it's doing the same exact thing. Weak signals have that clickety sound going constantly and I've noticed the volume and squelch don't have any effect on it. The squelch I expected but the volume is the part that gets me. If it was in the signal, the volume should change the loudness of the noise. I'm wired directly to battery with multiple ferrite bead filters on power and coax. Already tried with and without mic extension and tried a different antenna. Sound is present with engine running and without, also with vehicle completely off. Strong signals (when I have perfect LOS with the transmitter) don't have the noise, but everything else does. I also notice it when I turn the radio off. It's not present when listening to NOAA channels, however.