Crackling sound in Spider Forest plays even with master volume turned to zero

I'm doing a Spider Forest map and I have the master volume turned down to zero. (Unfortunately, I can't recall if I turned it down before or after I entered the map.)

All sounds are muted except for a constant crackling sound, which sounds like a part of the map's ambient sound. It doesn't sound like a sound bug or static, but like it's supposed to be crackling branches or skittering spiders or something.