Edward Stark

Ohh thats a normal operation of our tranny. Welcome to the club. Sometimes 4L60e goes powerstroke on you.
That was the Pump rotor, Forward sprag, 4 pinion planetary or maybe the input shaft.
This requires dissasembly and inspection.

Drop the tranny and tear it down, Trust me, you'll find whats wrong with it.
If youre on a good budget, Just order a built and upgraded one from Mr Frank @performabuilt .

If you want to build it yourself, there are tons of guides here. Years of searches and you will succeed building it.
But before you do that, go to youtube and search for Transmission bench channel, put 2x speed on the playback to save yourself a few months of life time and learn all his 4l60e videos.

bbond105

You haven't given much information but most likely the forward sprag broke and the trans will have to be torn down to be cleaned out and repaired. At that point you may as well just rebuild the whole thing, since you will have it all apart anyway.