Shifting to 4H noise

[Alcotune]

The last few days its been very cold and when shifting from 2H to 4H there is a noticeable clunk. I've only had it in 4wd a handful of times and never noticed this before. Is it normal to feel or hear the transfer or is this something I should have checked? I have roughly another month or 2 before my next service.
I've tried while parked and while moving (30kph) and get the same noise. This is on a base model with auto & 2.3 (no lockers).

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[thesocalexplorer]

I've always noticed this when switching over - like a light thunk, but nothing that sounds terrible or anything. I think it's normal operation.