Clean Bill of Health from Vet, but what about the popping sound?

[200xth]

I went ahead and bumped up the heat to make sure on top of the substrate reaches the high eighties-low 90s.

I, personally, would turn it down.

They can and will burrow. The hottest spot should be the bottom of the tub itself. If it's covered in substrate and they need it warmer, they will burrow to the heat on the tub floor. Just as an example, my tub floors are kept at 87 to 89, there's substrate on top of it, and they move things around in their tubs as they need to.

If your vet is good and is correct there is no RI, the popping noise could just be stuck shed around the nose area. I've had that happen before.