I've noticed the past few mornings when I start my 2000 F150 5.4L I hear a light knocking sound. It goes away after a minute or so. It will only knock in cold weather (Under 32*) on a cold start. Once the engine warms up, it is fine. I change my oil every 3K miles w/ 5W20 Motorcraft oil and a Motorcraft filter. Piston slap maybe?

Any input is appreciated.

 

Try changing to 2w-20 or even 0-20 and make sure ti use motorcraft anti drainback filters. I know ford calls for 5w-20 anyway.

 

I actually use 5w20. That was a misprint. Thanks for the input.

 

Piston slap!
Its very common on our engines. Mine does it also. don't worry about it.