The 6-speed manual in the 996 Carrera is the Getrag G96. These cars are over 20 years old now and a lot of these gearboxes are tired.
Before I open a gearbox, I make sure the gearbox is really the problem. A grind going into gear can be a clutch that does not release all the way. A sloppy shifter is often worn shift cables and bushings. These are cheaper fixes, so I check them first.
When it is the gearbox, the most common thing I find is the second gear synchro. You feel a grind going into second, worst when the oil is cold. A whine that changes with road speed is usually a bearing. A howl in every gear points to the pinion bearing. If you keep driving it, the metal goes through the whole case and the ring and pinion get damaged too. Ring and pinion come only as a matched set and they are not cheap.
Taking it apart is the easy part. Putting it back together is the real work. Pinion depth, differential preload and shaft preload are all set with shims, in hundredths of a millimeter. If it is not right, it comes apart again. I measure before I take it apart and I measure again during the build.
Some parts you can only get from Porsche. Some used parts are good and some are not. I check every gear, bearing and synchro hub, and I tell you what needs to be new and what can stay.
If your 996 grinds, whines or jumps out of gear, bring it. First I find out what it really is.
Also: Porsche service · Transmission repair