Q: I hear a rattling sound.

I have a strong suspicion that a timing chain guide has failed inside your engine. This is a pretty common occurrence with your particular vehicle. The timing chain guide are made of plastic and over time they get brittle and break. When this happens the timing chain will be looser than normal and if will make the rattling noise you describe. Unfortunately, to fix this condition the engine will have to be removed from your truck to gain access to the timing chains. I would suggest you ask for help from an experienced mechanic to properly diagnose where the noise is coming from and see if your timing chain needs to be replaced before getting too worried. It is possible that it could be something simple as well.