Shrieking Shack

From Hpwiki

The Shrieking Shack is a boarded-up house in Hogsmeade. There is a secret passage concealed under the Whomping Willow on the Hogwarts grounds which leads to the shack.

Supposedly haunted, in actuality it is where Remus Lupin spent nights while transformed into a werewolf when he was a student at Hogwarts.