Intermittent power LED and low speaker whine

I have this game gear which I just replaced main board and audio capacitors for and everything seemed okay following an hour or so of play testing, however one of the buttons wasn’t always responsive - so I opened everything back up, cleaned the button contacts with some IPA and closed it up again.

When I power it up now though, the power LED blinks on and off and the speakers are emitting a faint whine, changing its pitch in time with the LED’s flickering. I haven’t checked anything inside yet, but what could be causing this issue?