Never heard of such a case, but we have a hard time believing a transfer box that creates noise with an open clutch is able to function normally when operating.
I guess you are hearing a grinding noise from something in the rest of the drivetrain. Maybe a worn bearing or something touching because of a worn bushing. So something that gets amplified in RWD mode, but is already there anyway.
This needs to be checked by mechanic directly at car.
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