How to get rid of "CKK" Sound on "turn off"

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Have a "CKK" sound at outlet when turning off my DIY Dac with a slight thump heard through speakers. The sound does not occur at the Dac end, but at the other end of outlet...been testing this in the garage not in my system.

The Dac draws 120VAC tested. It has a 3 prong IEC inlet. Hot goes to separate fuse holder and then onto DPDT rotary switch. The switch is wired in the Euro way...where it switches both hot and neutral. Of course the IEC ground goes to chassis ground point.

I've been testing this over the course of a month. It has made the "CKK" sound at the outlet the whole time. It tripped the outlet once (GFCI) and I reset it. However, yesterday it tripped the outlet and it would not reset...I have no power from the outlet. The detached garage is served by a 30 amp breaker (separate feeder panel) from the mains panel.

Any thoughts on how to get rid of the turn off "CK" sound, and whether I have a bigger problem?
 
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