3.0 Caravan Lifter rattle

I recently rebuilt my 3.0 Mitsubishi engine and I get what sounds like bad lifter rattle once the engine warms up. I replaced the lifters and the problem perists. I even went to a heavier oil and addititives. Oil pressure is good. The rattle is the loudest when accelerating and the engine goes above 2000 rpm. Oil is flowing over all the rockers and I don't feel any play in them. It's like the lifters can't expand and contract fast enough at high RPMs to keep up with the Cams. Any ideas????? :confused: :confused:

The only thought that comes to mind is , what kind of head work was done during rebuild? I am think about valve stem height.