Sounds like ball bearings in a metal coffee can on startup for approx 3 seconds then goes away. Sounds like it's coming from the passenger side from under the hood. Hard to pinpoint as I'm alone and only happens at startup so I can only listen for it when I turn the ignition over. Truck is a 12' 6.7. Any ideas?

 

Loose heat shield?

 

Probably not a heat shield, only because that would continue to rattle until you wacked it (had to do it on mine two days ago). Doesn't hurt to check them though.

Check your air filter box for nuts; squirrels love sticking them there. I have to clean some out each year, but I've never heard them rattle. Never know though.

 

I would guess flexplate to converter bolts loose but it would happen every stop sign or light afterwards. And I can not imagine a scenario like that except for an oops moment after a transmission install or something.

 

My heat shield rattles everytime I reach the same RPM.

 

my guess is oil is too thin and the lifter is not able to pump up until the oil molecules start to curl . if you are runing 5W40 or 10W30, try a 15W40 oil.