Suddenly (αφνω). Old adverb, but in the N.T. only in Acts (Acts 2:2; Acts 16:26; Acts 28:6). Kin to εξαιφνης (Acts 22:61).

A sound (ηχος). Our εχο. Old word, already in Luke 4:37 for rumour and Luke 21:25 for the roar of the sea. It was not wind, but a roar or reverberation "as of the rushing of a mighty wind" (ωσπερ φερομενης πνοης βιαιας). This is not a strict translation nor is it the genitive absolute. It was "an echoing sound as of a mighty wind borne violently" (or rushing along like the whirr of a tornado). Πνοη (wind) is used here (in the N.T. only here and Acts 17:25 though old word) probably because of the use of πνευμα in verse Acts 2:4 of the Holy Spirit. In John 3:5-8 πνευμα occurs for both wind and Spirit.

Filled (επληρωσεν). "As a bath is filled with water, that they might be baptized with the Holy Ghost, in fulfilment of Acts 1:5" (Canon Cook).

They were sitting (ησαν καθημενο). Periphrastic imperfect middle of καθημα.