Audible interference on Sony CMT-EX1

Acquired a CMT-EX1 a few weeks ago, but quickly realised there is audible interference from the speakers, in one channel more than the other. Have opened it up, and I’m getting interference right back to the outputs on the amp chip. However, the digital-sounding interference is independent to volume, which baffles me… if the issue is back as far as the amp board or before, wouldn’t changing volume amplify the interference as well? Amp board 15V input reads 15.43V and capacitors don’t look bulged. RCA outputs have no interference. I’m in EU so there’s no earth ground, but I’ve tried different sockets/outlets around the house.
Where should I be looking next? Thanks in advance.

Audio:
https://voca.ro/1lQXB9ILOx9t
https://voca.ro/1imIFyhpLEAl
https://voca.ro/1gVTKyGYOz72

Schematics:
https://www.manualslib.com/manual/1216074/Sony-Hcd-Ex1.html