Dodgy relay or connection to relay. Unplug and reseat all relays after spraying the contacts with contact cleaner reseat. If problem persists you should be able to hear which one is buzzing or feel by lightly touching hte casing. You may get away with dismantling the relay and cleaning the contacts. if contacts are fused try to snap them apart and clean contact with 1200 grit sandpaper is a good short term solution.
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