Dick, is that a switching power supply? I strongly suspect you've found the source of much noise.
Wrap that DC cord a couple turns through a big mix 31 ferrite. Do the same (with another mix 31 ferrite, not the same one) with the AC supply cord that feeds the power supply (and shorten that cord too, if you can).
Very low cost, easy to do, can't hurt anything, and has a good chance of helping.
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-- Carl
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