I've had that cold winter knock and so have many other members. And I was worried at first, but I've had the truck almost 4 years now, and it still knocks for about 3 minutes or so, and basically goes away (I hear slight knocks, could be from gas, etc) plus i have over 155k miles on her. As long as you checked the oil and it looks good, I wouldn't worry, just check periodically.