I have an I3 mega s and my printer makes the same noise on the z-axis when going in a straight line. I never really thought it was odd since it prints just fine. In my experience there isn't any correlation between filament and the noise, it happens with both PETG and PLA. But did notice that on some models it happened more than on other, probably because some models have more single axis paths than others.

It sort of sounds like a resonance thing to me. If you try speeding up or slowing down the print and it stops making that noise, it's most likely resonance. Are you able to get an idea of where the noise seems to be coming from? Like the gantry or base, etc. I also know people have been putting automotive sound deadener inside the enclosure on the bottom panel as it seems to make a lot of noise from resonance.

Also, I am planning on purchasing some more hyper PLA to see if I can observe the same buzzing with that filament. I can’t imagine the filament type would cause the buzzing, but strange things do happen. Will report back here with any findings on that, but if anyone else has any thoughts in the meantime on this buzzing, I’m very keen to hear them!