It only has 134K miles. I added lower gears and a 5-Star SCT tuner and since then I have what sounds like a clanky valve... especially at cold start up. It sounds like it's coming from one cylinder only. Once it warms up it's harder to detect except at light throttle/load at low speed.
Any ideas?

 

What brand of oil filter are you using, and what weight oil? Check for exhaust leaks at the manifolds.

 

motorcraft 10-30, but the guy at the shop thinks it's piston slap since the tuner has probably advanced the spark.

 

Piston slap was a somewhat common issue with early 5.4's and V10's.