trying to talk and grind his teeth at once
What sounds like Lenz chewing gum is really Lenz trying to talk and grind his teeth together at the same time.
Context
The stimulant's tell rendered as a small sound: the ceaseless jaw-grind mistaken for gum-chewing, the body running the drug's motor while the mouth runs the drug's monologue. The physiology and the compulsive talk are the same event, cocaine visible in the jaw before it is audible in the confabulation.