To creak is to make a high, groaning sound, like a rusty gate swinging shut. The old, worn floorboards in your house might creak as you walk down the hall.
Old doors and gates creak as they open, and tree branches creak as they blow around in very heavy wind. The sound itself is also a creak: "The creak of the front door in the silent house made them jump." In the 14th century, to creak was to "utter a harsh cry," and soon afterward it came to mean the same noise made by an object. Creak is imitative — the word itself sounds like a creak.