I got a SL2 from a guy outside of our navy base and it ran great for 2 weeks aside from needing spark plugs. This past Friday I’m coming back into base and down shift from 4 to 3, I guess clipping 5th gear cause there was a slight pop and when I shifted up from 4th to 5th I learned I lost 5th. Now when I put it in 5th gear, the shifter goes in the slot with no issues as if its functioning normally, but when I let off the clutch there’s a horrible whinining noise that’s extremely loud coming from the drivers side of the Saturn. It’s like something is spinning violently in there. It’s as if there’s a space between something in the clutch that’s not grabbing, like there’s rotation but nothing locking in, if that makes sense. I’m illiterate when it comes to terminology, a pressure plate? Flywheel? Throw out bearing? I went and purchased everything I could find on a transmission that may cause the issue. Parts should be here by mid week. My question is, am I even going in the right direction?
Also when in neutral if you’re idling there’s a more quiet but similar sound when the clutch isn’t engaged. However if I press it in the noise stops.
Sluggish acceleration compared to the RPMs. All of this is leading me to think it’s the clutch, but I’m not a mechanic. There’s telltale signs it could be the input shaft as well. Is there any definitive way to test it? Aside from throwing on the new clutch and the other assortment of goodies I purchased.

 

As best I can figure from your description, I don't think your clutch is bad -- you can apparently still move without excessive noise in 3rd and 4th -- but I think 5th gear, or it's bearing or synchro is bad. The quieter sound you hear when engaging/disengaging the clutch at idle is probably the clutch release bearing (throw out bearing) and I would guess that sound is not new, you just didn't notice it before.

You say you ordered a bunch of parts for the clutch and transmission. Are you a good mechanic with a lot of experience in rebuilding manual transmissions for front wheel drive vehicles? (There aren't many good mechanics who will tackle that job.) My recommendation? Find a used transmission and replace the bad one, changing out the flywheel, pressure plate, clutch, and release bearing at the same time.

 

I'd have to agree with old marine. A salvage yard transmission with the new parts you have will be much easier than trying to rebuild the one you have. Of course, I'm basing that on my "experience" with transmissions. I can rebuild an engine, but I don't mess with transmissions. They're scary.