Double Thumb Question

[AccolaStudios]

As I've been learning to double thumb, I've noticed an issue when thumbing the D string. When my thumb goes through the D to rest on the G, I get a loud 'ringy' sound, which I believe to be the string hitting the last fret. Any ideas on what I'm doing wrong?

[AccolaStudios]

After closer examination, I realized it's a combination of hitting the last fret, and my thumb striking through and hitting the string off the fret board. Any ideas how to correct this?