A sound Perhaps this was a kind of thunder, accompanied with a great wind, which filled with terror and awe the whole company, and disposed them to receive the gift of heaven with humility and fervour. This noise appears to have been heard over a great part of the city, and to have gathered together a great crowd, who came to learn the cause. This noise and wind were symbols of the divinity. It was thus also that formerly on Mount Sinai, thunder and lightning, the dark cloud, the smoking mountain marked the majesty of God. (Calmet)
Jesus Christ, our Pasch, to answer perfectly the figure, was offered on the day of the great Jewish passover; so fifty days after, for accomplishing the like figure of the law given on Mount Sinai, He sent down the Holy Spirit on the day of their Pentecost, which meaneth fifty. But our feasts, as St. Augustine remarks, besides the remembrance of benefits past, contain great mysteries also of the life to come. (Ep. cxix. chap. 16.)
As of a rushing mighty wind:
this betokens the exceeding vehemence of the Spirit, but not for the apostles’ sake, but because of the Jews who were then present. Observe how it is always, "as of"; and rightly: that you may have no gross sensible notions of the Spirit, therefore it was not a wind.