RRR is on it about piston slap. Whether that's what your issue is depends on the build and the clearances of course, and how much it quiets (if at all) when hot, but if it helps any
mine is a 600-neighborhood forged-piston RB stroker, set up a little on the 'loose' side....and it has some piston slap. It makes a noise very similar to yours when it's cold....and it takes some driving and heat soak to really get the engine hot and tame it (cooling system almost works too well in colder air!)...
I had this clip of mine at a hot idle, you can barely hear that lower-frequency tap with the right ears but it's quite a bit more noticeable when cold---
