Is it related to when you depress/release the clutch? The type of noise you describe is typically the throwout bearing. However, the throwout bearing usually make noise when you depress the clutch and then stop when the clutch is released.
If there's insufficient oil and it's making noise, the damage is likely done. It appears the first gear to fail when you have low oil is 4th gear. If that gear is suspect, then you may have oil relate failure.
Jeff
Bellevue WA
90 535iM - not much stock remains. 3.7 liters, ported head, cammed, 3.73 diffy, M5 brakes, MAFed, yadda yadda yadda
86 Porsche 951 - Track Toy