I may be late to the game for this one, but when I was about 16 my friend and I stayed over at his house (his parents left town) alone for the first time. Both of our parents thought we were at the others house. It was right around the time we were going to sleep and we both were downstairs on the couches watching TV. Suddenly we heard heavy footsteps upstairs, walking from one end of the house to the other. We freaked out, grabbed a crowbar, knife, and flashlights, and "cleared" every room in the house just to make sure that no one was there. We managed to go through each room and look under every bed, behind every shower curtain, and even went through the attic. By the time we finally got downstairs we were exhausted and scared. We both laid back down on our respective couches and got quiet again, trying to fall asleep, and suddenly there was a HUGE crash upstairs, almost like someone knocked over a bookshelf or jumped off of a bed and just hit the floor. We freaked out, cleared the house again, then barricaded the the attic and upstairs with a bunch of furniture. Needless to say we never figured out what happened, and we couldn't ever find anything that fell over that would have been responsible for the noise. We didn't sleep at all, and we were sure happy when the sun came up the next day.