has there ever been a whistleblower about something going on involving the acting president?
I think US presidents, for the most part, try to insulate themselves from directly illegal activity. For instance, Ulysses S Grant doesn't seem to be involved with the Whiskey Ring, though his long time friend and secretary was at the heart of the matter.
But there are exceptions to this in more modern times.
Arthur S. Moreau Jr. (probably) orchestrated leaks to raise awareness of Iran-Contra. Reagan was almost certainly aware of trading arms for hostages.
William Mark Felt, Sr worked with investigative journalists to expose Nixon's involvement with illegal activities.
This time it has become a very big deal, but has anything similar ever happened that simply did not become known to the general American public at the time?
Not quite whistle-blowing, but Nixon prolonged the Vietnam war at the cost of 22,000 American lives in order to win the election. President Johnson was aware of this at the time but didn't publicize it. This story is quite old, but new tape evidence came up a few years ago, making this effectively buried for ~ 45 years.
Benjamin Franklin was a whistleblower