knock at the door
knock on the door

Even if they knocked at my door, I wouldn't tell them a thing.

But to make my mother happy, I knocked at their door.

She moved away, and I knocked again at the door.

He knocked at the door, and the man told him to come in.

They just knocked at the door and said how would you care to take part?

Then he went up the front walk and knocked at the door.

A few minutes later someone knocked at the door again.

A few minutes later we knocked at the door and he let us in.

One of them went ahead and knocked at the door.

When I knocked at the door, he told me to come in.