I was changing out my tensioner and belt on my truck because of a noise I heard that I thought was coming from there.Turns out the noise is coming from my under the valve cover on the passenger side.Its not a constant noise.Its like a rattling sound.Any ideas?

 

Are you sure it isnt the pcv valve, because they sometimes make a rattling noise, if its not that i would say a rocker arm came loose. Just pull the valve cover an take look and shake all the rockers an see if any arereally loose