That's certainly a pickle. Assuming that nothing is plugged into M/L inputs while this is buzzing? I'd unplug everything and see if this still happens. If it it disappears, reconnect one at a time until it comes back to find the culprit. It can also be a bad capacitor somewhere common to all M/L inputs.
Is this a new problem that developed or is this DM24 a new acquisition and you discovered this issue right out of the gate?