i've noticed that if i lightly press the pedal and only let the engine rpms get up to around 750-800 and hold it at those rpms the engine starts producing a distinct slapping sound. Sounds like front driver part of engine.
goes away if i get it past 800 rpms.

it makes the slap sound weather its in park or drive at 750 rpms.

Any suggestions on what it is or whether it will hurt my truck. A trip to the dealer may be in store.

Its a 2005 S-cab 5.4L 4x4 with 34,900 miles.

 

cam phaser mine been doing it since 34k no probs yet. got 50k now

 

Mine does it too, at around 800-1000 RPM. Took it to the dealer about 20,000 miles ago about it. They replaced the cam phasers, but that didn't do any good. I've learned to live with it. It doesn't seem to affect performance at all.

 

check the allingment of the exhaust and the cat, i took my to the dealer today for it and they realligned it and its fixed it.