I have a 97 with a 4.6L. When the truck is in park and you rev the engine from under the hood, I hear a rattle coming from the engine. You can hear it when you accelerate the engine but more when it decelerates. To me, it sounds like it is coming from under the valve covers. I can hear it more from the passenger side. I am going to be taking it on a 12 hour trip this weekend and now this is bothering me. Thanks for any help!!!

 

by the way, the truck has 107,000 miles and I have been using mobil 1 since 70,000. The truck is very very well maintained. Thanks!!!

 

Check on the passenger side valve cover, toward the back of the motor. On my 5.4 I have a...something...that comes out of the valve cover in that location that makes a crazy ticking/rattling sound like you are talking about. Was actually going to take a pic and post about it tomorrow. Its about a hand's length from the oil fill tube toward the back of the motor on my 5.4
Not sure what it is but wanted to see if maybe we were hearing the same thing.

 

Are you talking about the pcv valve?

 

Could be. I havent bothered to look it up in my Haynes yet. I just know its loud and could be the same sound you were talking about