Josephus - Antiquities of the Jews Book 1 chap 3:1
"(73) for many angels of God accompanied with women and begat sons that proved unjust, and despisers of all that was good, on account of the confidence they had in their own strenght: for the tradition is, that these men did what resembled the acts of those whom the Grecians call giants.
(74) But Noah was very uneasy at what they did; and, being displeased at their conduct, persuaded them to change their dispositions and their acts for the better; - but, seeing that they did not yeild to him, but were slaves to their wicked pleasures, he was afraid they would kill him, together with his wife and children, and those they had married; so he departed out of that land.

footnote: this notion that the fallen angels were, in some sense the fathers of the old giants was the constant opinion of antiquity."

(according to the ninth century writer Hiwi al Balkhi the Nephilim were the builders of the Tower of Babel. [R.C. Saadiah. Polemic against Hiwi al Balkhi pp. 54-56]

In the Companion Bible, Ap. 25. DR, Bullinger says:
"But we read of the Nephilim again in [Num 13:33] 'there we saw the Nephilim, the sons of Anak, which come of the Nephilim.' How, it may be asked, could this be, if they were all destroyed in the Flood? The answer is contained in [Gen 6:4], where we read: 'There were Nephilim in the earth in those days (i.e. in the days of Noah); and also AFTER THAT, when the sons of God came in unto the daughters of men, and they bare children to them, the same became [the] mighty men (Heb. gibbor, the heroes which were of old, men of renown)."

(So after this first happening there may have been a second time that this occured. According to Dr. Bullinger these people were also known as the Rephaim or Emin. They were also named among the Caananite people.
Rephaim is often translated as 'dead', 'deceased' or 'giants'. In [Isa 26:14] these Rephaim are said to have no resurrection.
The name Rephaim is translated 'dead' seven times in the Bible [Job 26:5; Ps 88:10; Prov 2:18; 9:18; 21:16; Isa 14:8; 26:19]
'deceased' in [Isa 26:14]
As its proper name ten times [Gen 14:5; 15:20; Josh 12:15 (marg); II Sam 5:18,22; 23:13; I Chron 11:15; 14:9; 20:4 (marg); Isa 17:5]
'giants' all other places.)

(The principal locality of the Rephiam was Astoreth Karnaim, and the Ermin were in the plain of Kirathaim.)

The Companion Bible states in Ap. 23 that:
"It is only by the Divine specific act of creation that any created being can be called a 'son of God.' Hence Adam is called a son of God in [Luke 3:38]. Those 'in Christ' having 'the new nature' which is by the direct creation of God [II Cor 5:17; Eph 2:10] can be, and are called 'sons of God' [John 1:13; Rom 8:14,15; I John 3:1]