seems it only does it while walking on the gray/fractured rocks

How did you step onto the rocks?

DayZ Monolith rocks do not have a road surface configured.

This causes the audio engine to play the last known surface audio before stepping onto it.
So for instance, if you took your shoes off before getting on them, your steps would be quite even with boots.
Step on sheet metal before, entire rock will be sheet metal.

It's an ugly bug that exists on a LOT of dayz assets.

I discovered it on my map a few months ago when I placed one of the Sakhal mobile labs next to a rock and steped off onto one.