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Clicking noise?
Mine make no sound (besides the switches clicking) but a have an old mega-s. Your seams to be a mega (older than mine).
Test your z stops by elevating the gantry like 5cm or more, than switching each one with your fingers (with not a so long interval, otherwise the gantry will continue to drop at one side). If it activates with your finger, you have to adjust the z screws. If not, the problem may be on the switch itself or its connection to the main board. Finally, exchange the switches and see if it works on the other side. If it does, cabble connections or board.