Knocking on a cold crank

[stingray72]

My truck knocks pretty hard in the morning when I crank my truck. It's only on a cold start and only when the truck is idleing. When I give it gas the knocking stops and after a few seconds of warm up it doesn't knock while it's idleing anymore. This is a pretty loud knock and you can feel it shake the truck

Anyone have any ideas as to what this could be so I can start figuring out how to fix this problem.

Thanks