Infiniti G35: SLIP function graoning?

I have a 2003 G35 sedan. It is RWD only, so it has the SLIP function to help guide it on slippery road.

I've noticed lately that when I'm driving on snow, for instance, and the SLIP engages, it also making a sort of groaning or creaking sound. I did not notice this last year, my first winter with the car, but I suppose it's possible that this sound got by me.

Is this sound normal? If not what might be causing it, and is it a warning sign for my car?

Thanks.

Chuck