«صَكَّةُ مُسْتَفْتِحٍ»
The knock of someone seeking to have a door opened.
Mustaftiḥ is the active participle of istaftaḥa, to seek an opening. The noun ṣakka conveys the knock itself and its modifier gives the purpose.
Mustanbiḥ and mustaftiḥ share a derived pattern: the first seeks a response, the second admission through the door.