Have you tried different interconnect cables?
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Here is one of a number of prior threads that have addressed this kind of problem, in which several approaches are suggested that you may want to try in addition to those suggested above.
In that case, as you will see, the problem was ultimately resolved by connecting a braided ground strap between the chassis of the preamp and power amp and ground on the power conditioner the person was using. That may not be applicable in your situation, but experimenting with a ground strap connected between component chassis and/or ac safety ground (e.g., a screw on a wall outlet or power strip) could conceivably do the trick. It would provide connections that have very low impedance at rf frequencies (ordinary wire does not have low impedance at rf frequencies), hence making the different ground points in the system more "common" at those frequencies.