rap at

rap at something

 and rap on something
to tap on something to attract someone's attention. Who is that rapping at my door? Someone is rapping at the window, trying to get my attention. I will rap on her window and try to wake her.
See also: rap
References in classic literature ?
That is another point (replied he;) We must not pretend to determine on what motive the person may knock--tho' that someone DOES rap at the door, I am partly convinced.
The tea things were brought in, and already had Marianne been disappointed more than once by a rap at a neighbouring door, when a loud one was suddenly heard which could not be mistaken for one at any other house, Elinor felt secure of its announcing Willoughby's approach, and Marianne, starting up, moved towards the door.
Researchers evaluated plant-produced mixtures with high RAP content (25 and 40 percent), different binders (PG64-22 and the softer PG58-28), and a control mixture with no RAP The study improved the understanding of the performance characteristics of hot-mix asphalt (HMA) mixtures with RAP at high, intermediate, and low temperatures and examined plant-produced HMA mixtures containing RAP.
The 28-year-old Science Communication student at the University of Glamorgan has also been asked to perform his rap at the Science Museum in London on Wednesday evening.
Rap at best, she says, is a "petition," a plea from young people to look at their lives.