Most likely this sound was produced by their own equipment. Yet it was also never replicated either.
It is plausible that a device on the moon was broadcasting and the alien operators, been taken by surprise shut down the hardware and then made sure there was no repeat.
Or they entered a stray stream of magnetized particles and replicated the Saturn effect.
It is now 57 years later and our explanations have not resolved it. If anything the alien question is becoming better supported. Too many good men have stepped forward to report such a presence to be overly comfortable dismissing it This is not one report of oddities but one oddity..