strange off-idle clattering sound in my 350

[mattfranklin]

1978 truck motor in my '70.
Was told by the PO that the guy he bought it from said it had a "rebuilt" 1978 350 in it with a "mild cam" and had about 30k miles since the rebuild.

It seems to idle quietly.
Just off idle it makes what sounds like tappet clattering sounds.
At higher loads I can't seem to hear it.

I don't think it's spark knock, that would be a higher pitched clatter.
I don't think it's rod noise. That would be more of a boom boom boom, from my past experiences. But I could be wrong. It's happened before.

The plan to to test compression to make sure nothing is seriously wrong.
Then readjust the valve lifter preload (or whatever they call it with hydraulic cam). Then put in thicker exhaust manifold gaskets.

I have heard that exhaust leaks can cause a clattering sound. But I'm just not sure. It has newer shorty headers.

Any ideas or suggestions on an easier path to check things?