Why would a manual transmission in a 91 festiva pop out of gear and rattle at high RPMs?

Answer 1

When the transmission pops out of gear it usually means that there is something loose or worn out.

Could be that the clutch needs replacing.

The "rattle at high rpm" could indicate a worn bearing... either the clutch release bearing or the front bearing in the transmission

More than likely, it may well be that you've ran out of tranny fluid. That will cause the the tranny to pop out of gear, and make a sound that is similar to having rocks in the tranny. But, that awful sound doesn't need high rpms. It will make that sound at nearly any rpm.

Yep, I'm betting it's out or nearly out of fluid. You have a leak. You check and/or add fluid thru the speedometer cable hole. There's one bolt, about a ten millimeter, that holds the speedo cable in place. The hole cable goes into is also tapered, and it is tricky to get the cable out of the hole. You will probably have to pry it out. Be careful not to break the piece the bolt goes thru. A LITTLE pry bar, the type used for prying out tacks is the best device for getting the cable out of the hole. This type pry bar, if you can even call it a bar is the type that has a handle like a screw driver. I screw driver will also work, but it is harder. Good luck.