Noise when amplifier and chromecast share a power supply
If it is a ground loop, it's an AC ground loop....


Then, it's called "lack of common-mode rejection" and occurs because the isolating DC converter will have high frequency AC switching artifacts (noise) on both DC lines that power the chromecast. That in itself may not bother the chromecast but, given that the audio output from the chromecast will have a fraction of this noise superimposed on its audio output and that will cause the amplifier a problem.

That problem will be significantly worse when there is power supply sharing of the DC converter and amplifier.

If it isn't an AC ground loop...


Then it's likely that the DC converter is putting ripple and noise on the power line shared by the amplifier and then this is called "lack of power-supply rejection".