So far, the contenders to rename core are: noise, substratum, and keystone. I like substratum as a kind of synonym for core, but I think we don’t want a synonym here; a name specific to this particular layer seems better. The name keystone seems synonym-ish, but I might be missing an allusion. Noise is maybe too generic within the theme. How about rumble, which suggests a noise from below?

Ooh, I like that

I always appreciate the great analogies present in so many names for racket things :)

I like the rumble too, how about Spring? like a water spring, or as in “spring into action”

I like rumble it makes me think of mysterious subterranean activity