Knocks when started
 

I have a 2003 Ford Ranger with very low miles, I had just put on a new fabtech 3 inch lift kit on my truck. The lift consisted of front lowered spindles and add a leaf in the rear. When i first finished and put the truck back on the ground and i first started it, i heard a strange knock that happnens only when it kicks over and start. You can sometimes here it when you rev the engine a little. (The truck does not have any body lift)*. Does anyone have any idea what this could be? Or has the same problem?

You will just have to look around the engine and trans to see where it is hitting.

Put E-brake on then put into D or First(feather clutch), rev engine a bit, engine and trans should move a bit and make the knock noise.
Also try Reverse gear with same method, see if you can track it down.

Could even be some new part that didn't get tight enough, starting and reving engine torques frame a bit.

Is it a knock or a tick? Leaking manifolds and or exhaust will sound like a ticking noise under acceleration