I have the EXACT same noise but it only happens in the way you described under 10 degree weather give or take. Maybe others can whey in on this? When I first herd it I stopped, got out, and crawled under the xk but seen nothing out of the normal. Therefore, I stopped worrying about it and havnt since. I honestly cannot see it being anything too serious since its obvious it only happens in very cold weather.

Sent from my SCH-I535 using Tapatalk 2