Start with gear oil change with the factory recommended fluid. It could be clutch noise but a bad throwout bearing will show some change in the sound when you put some light pressure on the clutch petal.
If the noise is coming from inside the transmission, things will deteriorate pretty fast. Gears are “hardened” which is on the outside of the teeth. Once you wear through the hardened material the gear will wear out quickly. Meaning more noise and eventually a total failure. Bearings can last a bit longer, but once badly worn the noise will get louder and the shaft the bearing supports can move and take out more parts of the transmission.
I really don’t know your mechanical skill, but I hope you have a lot of skill, a very good manual, or a knowledgeable friend it you break apart the transmission. Taking it apart is easy, putting it back together is a whole different story.