knock at

knock at (something)

To strike something sharply and swiftly. Hush for a moment. I thought I heard someone knocking at the door. I was kept awake by the sound of tree branches knocking at the windows.
See also: knock
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.

knock at something

to knock [on something] at a particular location. I could hear someone knocking at the door next to mine. Who is knocking at the door?
See also: knock
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.
See also:
References in classic literature ?
In fact, they were all enjoying themselves mightily when they heard a knock at the outer door, which Sadie rose to open.
Passing down the street, the Caliph had been attracted by the music of instruments and the sound of laughter, and had ordered his vizir to go and knock at the door of the house, as he wished to enter.
THE knock at the door was repeated--a louder knock than before.
"I warned ye baith, it was a clean impossibility to knock at the door this time.
Raddle paused to listen whether the repetition of the taunt had roused her better half; and finding that it had not been successful, proceeded to descend the stairs with sobs innumerable; when there came a loud double knock at the street door; whereupon she burst into an hysterical fit of weeping, accompanied with dismal moans, which was prolonged until the knock had been repeated six times, when, in an uncontrollable burst of mental agony, she threw down all the umbrellas, and disappeared into the back parlour, closing the door after her with an awful crash.
Another knock at the door announced a large-headed young man in a black wig, who brought with him a scorbutic youth in a long stock.
Jardines that a knock at the front door is "treated as an invitation or license to attempt an entry," while a knock at the back door can be viewed as a Fourth Amendment search.<br />North Carolina's appellate courts have reached similar conclusions between 2012 and 2017 in State v.
Knock Knock (18, PS19.99) MARRIED dad Evan Webber (Keanu Reeves) is home alone when, during a storm, two soaked and lost girls Bel (Ana de Armas) and Genesis (Lorenza Izzo) knock at the door and ask for help.
I think he got a knock at the weekend but these Everton players, they're all fit which is good."
Predictably, after the decisions in Payton and Steagald, cases arose where officers made warrantless arrests of unwary suspects who responded to a knock at the door.