Car is a 99 ex, stock motor/tranny, 231k miles.

Not even 15 minutes ago I was leaving from a store and out of no where, I was rolling in first and my transmission started to whine ridiculously loud. It made the sound you hear when you're in reverse...except I was going forward. It scared the **** out of me. No sound at idle / stopped. I had to stop at a red light right when it started but as I went again, it started. The higher the speed, the higher the pitch. It got quieter at higher speeds. It seemed to sound the worst in the first couple gears and happened even in neutral as long as I was rolling. About 10 minutes later, poof, it just stopped within a split second, while I was moving. The transmission sat for just over an hour after being warmed up all the way, so it wasn't completely cold but pretty cool at that point.

Any ideas on what it could be? I know my input shaft bearing has been on it's way out for many years because when I push in / let out my clutch I can hear a slapping / grinding noise that comes and goes away (goes away with the clutch in).

 

From your descriptions, it def sounds like a input shaft bearing or so. I would drop a bearing and seal kit in to it and inspect everything while you have it apart.