Cam and lifter problem
The oil gets thinner after warm up and bleeds out faster than when it's cold. At idle, being pressure is low, the lifters could be squeezing out the oil, thus the clatter.
(This is just thinking out loud.)
My Rhoads clattered a bit. Went away at a few rpm's above idle. Perhaps the Comp lifters have a larger oiling hole in the lifter body?
(This is just thinking out loud. Again.)