There are some spam issues in the comments, so I had to disable comment editing for a short time. It'll be back once the spam bots are gone. FYI... they leave a comment with "great" and then edit in the spam stuff later, to get around the filter.
(For actual knocking on doors, "klopfen" is just as common, if not more so. This verb is more often used in a figurative sense. More idiomatic WITHOUT mentioning the object you're knocking on.)