The sign of the son of Man is an Angel preaching the Gospel of Jesus Christ to all the nations
 
Mat 24:30  And then shall appear the sign of the Son of man in heaven: and then shall all the tribes of the earth mourn, and they shall see the Son of man coming in the clouds of heaven with power and great glory.
 
Now back up a few verses
 
Mat 24:14  And this gospel of the kingdom shall be preached in all the world for a witness unto all nations; and then shall the end come.
Rev 14:6  And I saw another angel fly in the midst of heaven, having the everlasting gospel to preach unto them that dwell on the earth, and to every nation, and kindred, and tongue, and people,
 
Good Health, Warren.