Hey guys, it's been a while since I posted here, but I've got an issue I can't seem to figure out.

Truck is a 2008 F150 5.4 4x4. I changed the spark plugs the other night for the second time. Everything went smoothly. Got the truck put back together and cranked it up afterwards to let it run. Truck ran great, but as the engine temp got up around 150 degrees, I started hearing a very loud ringing sound. I shut off the engine, double checked all connections, removed the belt to rule out accessory drive noise. No luck. The noise stays constant regardless of engine RPM, so I decided to drive it. I found the noise fades away under load, then returns as soon as I let off the throttle. I attached a GoPro to a cross member near the transmission after narrowing the noise down to the bell housing area. Attached is a link to the video I made. I'd greatly appreciate any info or advice on the matter, as this is my daily driver.

 

I think it's a vacuum noise. Every time you open the throttle and the vacuum drops, the noise goes away. Maybe you took a hose off when messing with the plugs and didn't put it back.