knock

the sound of knocking (as on a door or in an engine or bearing); "the knocking grew louder"

negative criticism

a vigorous blow; "the sudden knock floored him"; "he took a bash right in his face"; "he got a bang on the head"

a bad experience; "the school of hard knocks"

the act of hitting vigorously; "he gave the table a whack"

deliver a sharp blow or push :"He knocked the glass clear across the room"

rap with the knuckles; "knock on the door"

knock against with force or violence; "My car bumped into the tree"

make light, repeated taps on a surface; "he was tapping his fingers on the table impatiently"

sound like a car engine that is firing too early; "the car pinged when I put in low-octane gasoline"; "The car pinked when the ignition was too far retarded"

find fault with; express criticism of; point out real or perceived flaws; "The paper criticized the new movie"; "Don't knock the food--it's free"