Rattle under the hood on the driver's side

I heard something coming under the hood like is rattling. Not super loud. The sound is coming from the driver side close to the wall. By the electric brake booster. But when the truck is in park. The noise is gone. I check under the hood. I've noticed that there is a relay box there and is loose. Could it be that?

 

Could be anything. Keep troubleshooting.

 

@JoeCOVA im sorry. Troubleshoot?

 

It could be anything. Based on what you have provided it would be next to impossible for anyone of us to provide insight. I would just keep looking and searching to see what it is. The Engine itself can make a rattle sound. If you can record a video and provide we might be able to diagnose the issue.

 

Post video on YouTube then post the link here...

 

Check to make sure there ain't no Rattlesnakes hiding by the firewall.
They like to hang out by the relay box. It is starting to get cold and
it is warm by the relay box. Poke around there with a screwdriver
until you hear the rattle noise and you know you are getting close.

 

I also forgot to mention. This just happened after I took my truck to the dealership to get my 10k oil change.

 

The same guy that forgot to tighten the wheel lugs is a jokester, and put the rattlesnake up in there. LOL

You ain't in California are you?

 

Lol