The moan makes John's groin twitch excitedly. He had no idea hearing Sherlock in this way would make him feel so excited. He never even thought something like this would ever happen between them. Although Mrs. Hudson always assumed, John was too shy to admit he had any feelings what so ever.
John's eagerness is expressed as he rubs his own hips down against Sherlock's leg, seeking his own friction. But he doesn't stop touching Sherlock. He continues, hoping to push his friend over the edge soon. He wants to hear what Sherlock sounds like when it happens.