«سَمِعْنَا مِنَ الْبَابِ نَبْأَةَ مُسْتَنْبِحٍ ثُمَّ تَلَتْهَا صَكَّةُ مُسْتَفْتِحٍ»
We heard a cry from the doorway, as if someone were calling to be noticed; then came a knock from someone seeking admission.
Nabʾa denotes a sound or cry, and ṣakka a knock or striking sound. Thumma establishes their sequence.
The narrator hears before he sees. That delayed identification heightens the small mystery at the door.