Looks like a piezo speaker. As it's close to the tape/metronome connectors, it might be just that for. You would have to look a the bottom side of the PCB to see if connects to those connectors.

a speaker for the metronome make sense! Will check the lines.

if the speaker is indeed the issue then you can just desolder one of it's legs , make sure it does not make contact and just leave it inside the box
that will shut it up 🤣

I had no idea these contained piezo speakers in them. I had an MPU-401 start buzzing when hooked up to a particular interface card and didn't know what might be causing it.

Yeah given the original break out boxes were designed for musicians rather than gamers or casual users they had features like a metronome and a din sync connector for drum machines plus tape sync.