Ok so here is the deal. I have a 2001 F150 4.6 138,000 miles that recently I have noticed that when I lightly accelerate I have a weird rattling sound (sounds like a spray paint can) coming from the passenger side of the engine but loud enough to hear in the cabin. When my truck in idling there is no sound. However if i am in park and rev the engine you can hear it as well only after the engine is warmed up. I have tried switching to 93 octane to see if it was a spark knock however it is still present but quieter. I have cleaned my MAF, replaced TPS, replaced IAC, Coil packs are fine and plugs are in tight. The sound is definitely coming from the passenger side of the engine, sounds are near the valve cover on that side. It is very specific when it happens. It is when my RPM's are around 1100-1300 and when I very lightly accelerate.

I am completely out of ideas out of this point. I haven't bought my tuner yet so I haven't been able to adjust certain things to see if it is timing related. The only engine/exhaust mods on the truck are: CAI and a Flowmaster muffler

I am also using 5W-20 fully synthetic as well.

 

Could you post a picture of the sound?

Describing only does so well sometimes.

 

Plugged PVC valve or modern day equivalent? Unvented crankcases are terminal...

 

Is the noise coming from the VC area or just that side . A bad cat can rattle . Just a thought .

 

I replaced the pvc valve already..the first thing i checked as well

 

I have another thought. I have narrowed i down to possibly 3 things.

1. one of my serpentine belt pulleys
2. timing chain guard or tensioner is broken
3. spun bearing

I don't think it is the spun bearing because the sound isn't constant and at this point if it was spun my engine should have probably seized.
I do think it may be the timing chain because based on what i have read all around on the forums it sounds consistent with my issue
I do think it also could be one of my pulleys as well as i haven't taken off my serpentine belt yet.

 

Easiest thing to do first is take the belt off.