Rattling noise from under my truck

I'm assuming you have a 4.6 V-8. Without more info it is difficult to help you. How many miles on it? If it has at least 100k on it it could be chain tensioners or guides.

check all pully's run off the serp. belt. had simular prob.(2003 f-150, 300,000 kil.) turned out to be tensionner arm and the steering pump. replaced both, prob. solved.