Silence.
It had become second nature to them since Ron's departure. Harry resigned himself to giving Hermione as much space as she needed and unfortunately this made Harry feel more alone than he's ever felt with just her.
One night he had hoped to break the stillness set between the pair of them, if even just for a second; he was re-entering the tent following his watch and as Hermione was about to pass him he embraced her on the spot and whispered, "Thank you."
He wished he could tell her more, but hopefully that was enough to begin again.