The loud sounds coming fromt he piano masked the sound of the door opening and closing again. That was why the Gryffindor wasn't realising that someone had entered the what he thought was empty room and continued to play. Aka, slamming the keys.

Whoever might hear the sound, they would probably thought that he was having a fight with the piano but he truly wasn't. He continued to play until he felt something. Or more like he had the feeling that someone was watching him. And then he heard a chuckle. Turning his head ever so slightly and trying to look behind him, he caught the sight of half a kid's body. He grinned as he continued to play with one hand, aka slamming the keys and with his other hand he carefully slipped his wand in to his hand.

He stopped playing as he twirled around in his seat and pointed his wand at whoever was behind him. Be it a student or a Professor, it did not matter to him. "STUPEFY!" he shouted and watched the red light fly towards the .....kid....