Microkorg crackling line outs

I’ve just acquired a Microkorg, and I’m having some trouble with the audio output. The line outs left and right both have a noticeable crackle on all patches, and the crackle is somewhat rhythmic in nature and changes depending on the sound and pitch of the note. This crackle does not appear on the headphone output, which has a clean and strong signal. Can anyone point me in the right direction for this particular issue? Thank you.