knocking

The sound or action of repeatedly hitting a surface, often a door.

"There was a persistent knocking from next door."

Making a sharp sound by hitting a surface, often a door.

"Someone is knocking at the front door."

Informally criticising someone or something, often unfairly or negatively.

"Don't keep knocking his efforts."

(Of an engine) making a rattling or thumping sound due to a mechanical fault.

"The old car's engine was knocking."