Suddenly (aphnō). Old adverb, but in the N.T. only in Acts (Act 2:2; Act 16:26; Act 28:6). Kin to exaiphnēs (Act 22:6).
A sound (ēchos). Our echo. Old word, already in Luk 4:37 for rumour and Luk 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” (hōsper pheromenēs pnoēs biaias). 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). Pnoē (wind) is used here (in the N.T. only here and Act 17:25 though old word) probably because of the use of pneuma in Act 2:4 of the Holy Spirit. In Joh 3:5-8 pneuma occurs for both wind and Spirit.
Filled (eplērōsen). “As a bath is filled with water, that they might be baptized with the Holy Ghost, in fulfilment of Act 1:5” (Canon Cook).
They were sitting (ēsan kathēmenoi). Periphrastic imperfect middle of kathēmai.