HELP! Metal rattling noise!
 

Hi! I just replaced my intake manifold on a 2007 F150 4.6L. Everything went well, truck drives good. Today, though, I got her to about 65 or 70 and I hear, what I can only describe as 2 pieces of thin metal slapping together? I can't make it repeat the noise in neutral or park by raving the engine. Only when I am driving and only at that speed? Any ideas? It sounds as if it is coming from the back left side of the engine, possibly? Sometimes I have a hard time narrowing down engine noises while driving?
Any help is much appreciated.
Thank you,
JH