On the trip to Erid, anytime Rocky's in another room (rare) and quiet for too long, Grace will come find him to make sure he's still there. To remind himself he's not alone. This evolves into comments and questions yelled across the ship. Not anything that really needs a response, Grace just needs proof of life. Rocky doesn't, Rocky can hear Grace just fine from the other side of the ship.
They spend years with just each other on that spaceship. By the time they get to Erid, Rocky is solidly in the habit of periodically chirping so Grace can hear him. Regularly sending out nonsense chimes and notes and rumbles. Grace always responds. His responses have also devolved into nonsense.
Habits are hard to break. They keep doing it anytime they're together, but also when they're apart. A few weeks or months after Rocky makes it home, Adrian finally cracks: "my love, why the fuck are you so noisy??"
