It's the xe (Xtreme Energy) part of the cam that's producing the problem. The ramps on those grinds are just this side of letting the edge of the lifter dig into the lobe. Change to a Magnum grind and the rattle should go away.

You can tell whether a cam is ground on the extreme pattern or not by subtracting the 0.050" duration from the advertised (0.006") duration. Anything 50 degrees or more (280/230 for instance) should be ok. Crane, I remember, used to grind all their cams with a 56 degrees difference.

The XE256H is 212/256 on the intake, a difference of only 44 degrees.