O2 hissing during volume change

So recently my O2 from @MayflowerElectronics has started hissing whenever I turn the volume knob. After cracking it open, there's nothing visually wrong with it. All the solder points look pretty good, and it doesn't appear to have any damaged parts.

Decided to try it running on batteries, and the problem is non-existent. Any ideas on why my amp (which has worked flawlessly up until now) is suddenly hissing when I change the volume on wall power? Maybe my power brick is taking a shit?

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