“…When [the angel] cried out, the voices of the seven thunders spoke. Now when the seven thunders spoke, I was about to write, but I heard a voice out of heaven say: ‘Seal up the things the seven thunders spoke, and do not write them down.'”
~Revelation 10:3,4

Who is represented by “the seven thunders”?
In other Bible passages, the sound of a thunderous voice is often times attributed to Jehovah. (Job 37:4,5; 40:9; Ps. 29:3; John 12:28,29)
The number seven is used figuratively to convey completeness. (Insight, “Numbers”)
So in this case, the “seven thunders” seem to allude to Jehovah speaking about the completion of his purpose.
This theory is encouraged by what the angel then declares to John: “There will be no delay any longer. But in the days when the seventh angel is about to blow his trumpet, the sacred secret that God declared as good news to his own slaves the prophets is indeed brought to a finish.” (Rev. 10:6,7)
But because John was not allowed to write down the announcement from “the seven thunders,” and then he was instructed to seal the blank scroll, one can only guess what Jehovah’s message was about.
In any case, it was not meant to be understood in our times.