Mt Bike mechanical disc brake making clicking sound?

Since the sound is only present when you have weight on the front wheel, my immediate thought is to look along the outer edge of your disc and see if there is anything making contact with the caliper. Its possible you may have bent the disc caliper slightly and its making edge contact under a load. It sounds benign and harmless. Unless your brakes are in serious disrepair, its almost impossible for a mechanical caliper to lock-up on you for any reason. They are designed to not engage 99,99% of the time.