The first thing that comes to mind is the 700R4s front pump. I did some addition research on a few forums and a few people also said mentioned a possible torque converter but those usually went along with a problem in reverse, which you didn't mention. So I'm more inclined to learn towards that front pump I mentioned.
Unfortunately there no way, that I know of at least, to diagnose that without physically pulling the trans and inspecting the pump.
If that is some thing you have the ability to do there are lots of very good videos online that you may want to follow along with. If not I would recommend taking it to a mechanic that you trust.
I hope this helps!
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