My truck at around 1200rpm knocks loud. My issue is the cam chains on the passenger side. I believe it's the phasers and probably broken tensioner. It's just off idle and when it's warm. Also lately on cold start ups. Been like this for about 6k miles. That's all I've put on it since I got the truck.. sadly in my case if it let's go, I'll be getting a replacement crate motor.

 

I think your mechanic is right. Sounds like an exhaust manifold leak to me... but then I've never heard a cam phaser knock. I have a 2V 4.6. The staining near the exhaust port really puts the nail in the coffin. Exhaust leak for sure. Good luck.

 

Just to close out this thread, bit the bullet and paid for the manifold replacement. Good lord does it sound nice now. I was developing an anxiety complex over this dumb sound for the last 9 months. It wasn't cheap, so I see why you'd try to do this job yourself. In my case, I couldn't really afford to screw it up myself if I got in trouble with it. The manifold itself ended up being cracked, so needed a new one, not just machining the old one.



Cracked F150 passenger exhaust manifold.

 

And here, I thought it was your horn.

 

I'm glad that you got the noise sorted out. I admit to being wrong. I'm just glad that you didn't do the manifold for nothing! haha good job