Actually, I'm not playing Hexxit. I'm playing Tekxit 2.0, but I know for a fact that it's from the Hexxit part of the modpack (I've played Tekkit extensively and my friends and I have never had this experience). There's no Tekxit forum so I just posted here, considering that it has to do with Hexxit.
It's not from a specific block. I meant that the sound is not just uniform over an area, such as a cave sound effect. It sounds louder as you approach the "source," which is when you stand on one block. The farther away you get from the "source," the quieter it gets. It's End Stone inside a Hexxit dungeon, if that helps.
I realize this is pretty vague, so if you have no idea I don't blame you.
Thanks for the reply!