Peter, sounds like we may have the same issue.
Just for fun, on the next flight try tuning in something like 118, and see how fast the tick is. Then something in the 130 range, and see if the rate changes.
I talked to lots of Garmin folks about this, and no one was able to tell me what is going on. Just the same old "send it in for repair" story-----and some crazy flat rate $$ charge
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I solved the problem by selecting my SL 30 as primary radio, and keeping the 40 for use as a freq "notepad"-----or the very occasional need to monitor a third freq.
I
suspect it is something inside the radio, that is generating a beat freq that is normally not audible, but some other external influence like bad ignition is causing the tick to become noticeable. i.e. the electronic widget that normally suppresses the tick is too busy fighting with some other externally generated noise, and the tick slips through. But then I sometimes have a rather active imagination
My tick did not occur until the 40 hour mark or so.