Intermittent (bad) knocking noise

OK, just rolled over 4K on the ODO and love, really love this truck. I am however very concerned about a loud knocking (crank type noise) That has only happened 3 times now. The first time it happened I shut the engine down and looked under the hood to make sure a cat or something wasnt in the fan area. The second time I just waited until it went away whick took about three minutes. The third time I shut it down and looked under the hood again. I really dont know what I was looking for but I figured what the heck. The noise goes away after 3 or 4 minutes and has only happened three times. Its happened twice with the original oil and once with the new Mobile1. I live in Napa CA so cold weather not an issue. I did stop by the dealership and he said they would have a very hard time pinning down the problem, I agreed... Anyone else notice this issue? Thanks much.

 

ive never had that problem in my yota but my wife has a kia sorento and i occassionaly will hear a knocking noise.....narrowed it down to the blower motor for the defroster....just a loose fan or something.