Knocking, sometimes...
 

Ok, I have an '03 F150 w/ the 4.2. And yes, I've had the manifold gaskets replaced.

Problem is, and I'm assuming a problem, maybe not... When the truck is warmed up, and IN drive (automatic), it knocks. If I put it in park, it stops, and if it's cold, it doesn't happen. I'm assuming that it doesn't knock when cold because it's idling higher... I'm idling around 450-600 when in drive, maybe 850-900 in park (warmed up).

I use Motorcraft 5w20, and the truck has 37k.

Any ideas?

Not exactly sure what you mean by "knocking". Do you mean the engine is idling to low and sounds like it is going to die? Or, do you mean you have a metallic knocking sound occuring when in drive?
Is the sound coming from the engine area, or the transmission area?
Need more info....