It almost sounds plastic to me, - or metal, -light gauge metal tho. Iduno. Maybe something to do with timing components ie, = chain, plastic tensioner may have broken or loose. Is it right at the front, - did you stethoscope exact location by chance ? Yea, not sure. (?)

 

It does sort of sound like an injector as well. Probably not, but stethoscoping would determine.

 

Sorry I was not able to get back on yesterday, but yes, this is what my buddies 99 F150 5.4 sounded like when he spun a rod bearing. You couldn't hear it at idle, but as the RPM goes up, that noise was present. My buddies sound came from the back passenger side of the engine and was a little louder than yours is, but by the time he brought the truck to me to check out, he'd already driven it that way for months. The only other thing I can suggest is checking the first 2 plugs on that side and make sure they are not about to be shot out of the head.

 

used the stethascope all over the engine, definitely coming from the under the cover that covers the cam, seems to be near the top. It is right at the front of the motor, on top under cover. The vid i posted sounds like it does in person, but not exact, it does sound metallic. Im puzzled, guess the only way im gonna find out is pull it apart. I was just hoping someone on here had heard this before. Thanks for all your responses, any other thoughts you have let me know, thks....

 

check out my thread "missing stumbling 5.4". In addition to all the other symptoms, it started to rattle like that at 1000-1700 RPM...although not as bad. I am pulling my timing cover on Sunday, and expect to find bad timing chain tensioners.

 

Mine has a bad rattle, but I know its a bad cat on the truck and it sounds much more hollow than that. It sounds like a valve train problem, rocker or something like that, maybe a busted spring