Very nasty rattling on start up- auto choke?

I hate bringing my truck to the shop and avoid it at any time possible. So my truck recently started making a real nasty noise under a certain condition... when it's cold out and I start the engine, if I put it in gear right away with my foot on the brake, something sounds like someone shaking a coffee can full of nuts and bolts. Seems to only happen when the engine is idling high upon warm up. It goes away in a few seconds when it warms up a bit. I originally thought it was a heat shield knocked loose last time I was off road so I never really worried about it, but last night I decided to try and find the culprit and it seems to be coming from the front of the engine behind the radiator.

I can not reproduce this sound when it's warm and it's really hard to locate because it goes away really quick. I will start it up when it's cold with the parking brake on, then immediately put in gear and run out to try and find the rattling and I have maybe 8-10 seconds before it's gone. But it is really prominent and loud when it happens.

Any ideas? I have a feeling that the dealer is not going to be able to find the rattle because it doesn't happen when it's warm. I have no check engine lights or anything.

 

Maybe something wrong with the emissions pump(air injection pump). It runs for about 10 seconds upon startup. Does it make noise when shutting it off? It also runs when turning off the truck.

 

You know, I will have to pay attention to that when I drive it next. I believe the noise at shut down is like it always has been, but I will confirm in about an hour when I go to lunch.

 

Does it do this with the heat on the off position?

 

Another good question. I leave the heat on, so when I start it next I will make sure it's off. It sure seems that it's something very close to radiator fan. It's hard to diagnose, but I am going to ask someone to help me at lunch.

 

The radiator fan also runs on startup for about 10 seconds.

 

So when I start my truck next, I will make sure the heater is off, I will open the hood and check to see if the noise goes away when the fan stops.

 

Are you sure the rattling or pinging, is not a quick pre-ignition or detonation taking place until the computer corrects the problem?

 

Someone swapped your motor with a 22RE and your timing chain guides broke.

/notserious

If you can, have a friend help you start the truck while it's cold you might be able to more easily diagnose the problem. If you go to the dealer, request they hold your truck overnight and replicate the problem when it starts up.

 

Actually, a broken timing chain guide could be a possibility. Pretty low chance, but a possibility nonetheless.

Another thought, if the intake tube through the inner fender is similar to the older models, it might be rubbing and causing the noise. Mine only made noise when cold, and when the engine was placed under load, the first time or two letting out the clutch. It makes a heck of a racket due to the resonance through the body.