To strike a surface noisily to attract attention, especially at a door. She knocked on the door before entering the room.
To collide with something, causing a sound or impact. He accidentally knocked the vase off the table.
To criticize or find fault with someone or something. He tends to knock every new idea that comes up in meetings.
To cause something to move or fall by striking it. The wind knocked the sign over during the storm.
To make a pounding noise, often due to mechanical issues. The engine started to knock, indicating a possible problem.