Piston slap is the piston skirt actually contacting the cylinder wall. You can think of it as the piston "rocking" around the piston pin as the piston travels up and down the cylinder.

As far as the question of whether or not it will hurt your motor - I have heard of durability engines with 150k+ miles on them that slapped since new and were torn down to find nothing more than minor scuffing on the skirt. You decide, but in my opinion NO metal-to-metal contact is a good thing.

 

I also have a 2002 f150 5.4 that I noticed the piston slap about 3 weeks ago.. Doesnt happen all the time just when the truck sits for a long time 10-14 hours on a cold day.. The first time I herd it I blew it off thinking I was hearing things...Than I herd three times in a row coming out of work....

Anyhow I am a mechanic and yes its piston slap...


MBTECH....

 

Oh well. It doesn't harm the engine, no worry.