Noun

1. The sound of knocking (as on a door or in an engine or bearing)
2. Negative criticism
3. A vigorous blow
4. A bad experience
5. The act of hitting vigorously

Verb

1. Deliver a sharp blow or push :
2. Rap with the knuckles
3. Knock against with force or violence
4. Make light, repeated taps on a surface
5. Sound like a car engine that is firing too early
6. Find fault with; express criticism of; point out real or perceived flaws