Ground the metal rack,via the third prong on an AC cord.Make sure to contact the grounded rack and discharge yourself.
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The DAC probably has a mute when it loses sync. Try Toslink?
FWIW - Ground Loops are a common problem particularly as most audio gear uses ground for the signal (RCA) and many power supplies are leaky to ground - resulting in an imbalance in the signal wires with respect to a ground/shield signal. Even a modest resistance in a shield can induce signals in your interconnects with mere microamps of ground loop (the difference in ground between one socket and another in a house).
In your situation I would go all XLR with properly grounded gear (three prong).
Holding a quarter between your fingers and then touching the rack with the quarter to make that ground contact before you touch your equipment helps with coping with the snap crackle pop ouch!
Unless, of course, shocking yourself into attention before you listen to music is a requirement for enjoyment....
Ground the chassis of the DAC via the power cord. Ensure that all of your chassis grounds are made to the same point. That is, don't use separate ground wire runs back to your panel. If you have shielded interconnects, attach only one end of the shield to chassis ground, float the other end, in each case float the end at the forward part of the path (i.e. between the preamp and the amp, float the amp end).