I've always had suspicions about the bottom end in my truck. After warming up I can hear a slight knock in the motor at idle. I also hear a knock when i shut it down after running at operating temp, kinda like a clanking. Its been doing this for at least 3-4 years (maybe longer) and nothing has really changed that I can tell. I'm religious about oil changes every 3000 miles. As a matter of fact I changed my oil today (400 miles over) and noticed a few metallic flakes in my oil. I've been doing alot of freeway driving and some towing since the last change and just a few days ago I gave it the seafoam treatment (1/3 can in crank case). Just wondering if I should start to worry and if any one else notices this slight knocking I'm talking about in their motor. Feel free to set my mind at ease... thanks.

 

Mine knocks at idle but its exhaust related. It sounds like yours is too since it does it after you shut it off.