sealed power lifts

I assume the lifters have the oil holes in the tops, where the pushrods sit? And do the pushrods have the oil holes through them? (Sorry for all the dumb questions.... just trying to see if all is as it should be and the Magnums oil the rockers up through the lifters and then the pushrods.)

It could well be that this cam, with the probable heavy springs that go with it, are too much for stock hydraulics to hold up consistently with the idle oil pressures. I am assuming you are using a 5W30 oil? I'd try a 5W40 or 10W40, non-synthetic; the 40W is to make sure the oil stays heavier when hot and see what it does; if it quiets things, then that probably points to the lifters.

Stock Magnum rockers? That is a lot of lift with stock rockers IMHO, and the rocker geometry may be having trouble handling it. We don't really know anything about the rocker/spring setup here.

As you can see from that linked thread, hydraulics are less reliable than they used to be, and you appear to be pushing stockish ones pretty hard.