Knocking 'at' vs. Knocking 'on' [closed]

Which is more natural and why: 'knocking on the door' or 'knocking at the door'?

And which is grammatically correct?

When someone is "knocking on the door" they are striking it (with their knuckles or with the door knocker) to produce the sound.

When someone is "knocking at the door" there is the added nuance that someone is presently standing by the door and knocking on it.

at conveys the notion of presence-there.

There's someone at the customer service window, complaining about cheaply made products.

English uses 'on' for various relationships, including 'physically touching', even the if the surface being touched is vertical. Compare 'picture ON the wall'.