If you accidentally knocked on the wall of your room, and you heard a voice from the wall say "

....what would you think/do?

If you accidentally knocked on the wall of your room, and you heard a voice from the wall say "yes?"...?opera music

i would be shocked... and scared.. but try to carry on a conversation with the voice..

If you accidentally knocked on the wall of your room, and you heard a voice from the wall say "yes?"...?opera sheet music opera theater

GREMLINS!!!!

That'd be pretty sweet... I'd probably knock again. Just to make sure I wasn't imagining it. And then again and again and again, for pure entertainment value.
ill' say hello whats' up then look around and scream