It is, indeed, a gas line. And the pic doesn't really show the detail well enough. There is a good 1/8" clearance between the backerboard the the line itself.
Back to the gritty noise...
I just spoke with one of the more knowledgeable guys at the tile store about it. He suggested that perhaps I didn't get quite enough thinset in place where the gritty noises are coming from. He said this could cause the thinset that was there to start crumbling.
He also suggested this shouldn't be an issue, and that the gritty noise should go away as I walk on the areas, crushing the thinset.
The floorboards underneath are in good shape, secured down nice and tight. The backerboard sheets are firmly attached (currently with 56 nails per, but soon to be 88.
) So, once the follow-up nailing is complete, I should be starting with laying the slate in this area.
Randy