Ok guys, it's been a while but I need some help.
I'm looking at an 09 f150 with the 5.4L with 225k miles. I know it's a lot but the body and interior are dang near perfect.

My problem is that after the engine gets warm it develops a rattle under light acceleration. It is most prevalent between 1100 and 1500 rpms. With help I was able to listen under the hood and it's definitely up front and high on driver side.

Has anyone encountered this problem before or know what direction I should go in? It seems like when the oil thins the rattle begins. I almost want to switch to a 10w30 and see if the rattle persists.

 

Take a pass. You are looking at an engine on its last legs. You would probably need a complete timing component and oil pump replacement. Even then, the engine may have enough wear that it won't hold high enough oil pressure.

 

Yep, I'd pass too. glc is right.

 

Well damn, I guess I will definitely need to pass. I'm not trying to get rid of my reliable 2003 5.4 to get something I can't trust.
While I'm on the subject I found a 2014 stx 4x4 screw for 12000 with 213k on the clock. Looks to be in good shape but I haven't driven it yet.

 

That's going to be a much better bet.

 

Sounds like a cam phaser problem. With that many miles the whole timing assy (chains, sprockets, phasers, etc.) should be changed. That won't be cheap.