Zeke,
Welcome to Pelican
You will find a lot of help here.
The ‘singing’ is probably not a good song.
Typically, this can be a worn pinion gear.
The ring and pinion (R&P) gear-pair is very expensive new – but available.
Replacing it (even with a used one) is not usually a DIY procedure due to the special tools and experience necessary.
You might source a ‘known good’ used transmission from a Pelican. There are some available.
You will usually find ones with worn shifting pieces (syncros, etc.) Repairing this can be DIY.
You might want to get several transmissions and rebuild yours with ‘all the best parts’.
There are quite a number of subtle changes among these 901/911 years so consult here.
Tell us more about your 911 (flattering pictures necessary).
If your chassis-engine-transmission are as original, it is useful to retain your original (number) transmission casting.
Again, WELCOME.
Best,
Grady