{Suddenly} (aphn(935c)). Old adverb, but in the N.T. only in
Acts ( 2:2  16:26  28:6 ). Kin to exaiphn(8873) ( Ac 22:61 ). {A
sound} ((8863)hos). Our echo. Old word, already in  Lu 4:37  for
rumour and  Lu 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"
(h(9373)per pheromen(8873) pno(8873) 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). Pno(885c) (wind) is used here (in the N.T.
only here and  17:25  though old word) probably because of the
use of pneuma in verse  4  of the Holy Spirit. In  Joh 3:5-8 
pneuma occurs for both wind and Spirit. {Filled} (epl(8872)(9373)en).
"As a bath is filled with water, that they might be baptized with
the Holy Ghost, in fulfilment of  Ac 1:5 " (Canon Cook). {They
were sitting} ((8873)an kath(886d)enoi). Periphrastic imperfect middle
of kath(886d)ai.