Other forms: thunks
A thunk is a loud, deep sound, like the thunk your head makes against your desk when you fall asleep in math class. Again.
The informal, imitative thunk is a good word for the hollow thudding sound of something heavy making a dull impact. You can talk about the thunk of a door closing or the the thunk your heavy book makes when it falls to the floor. A second informal meaning of thunk is the humorous past tense of think: "Who'd've thunk I'd catch a fish with a piece of cheese and some dental floss?"