If topping off ,or replacing your oil does not help ,you could have a bearing going bad inside the gearbox. Usually it is the one on the end of the shaft ,that the motor connects to.
As stated above the gearbox could last a long time making noise ,but it could also fail tomorrow.So you may want to consider replacing it ,then rebuild the one you remove from service. You can get all the bearings ,and oil seals at any good bearing supply place. You will then have a spare for the next time.
We rebuild all gearboxes we remove, the only time we don't is if the gear,or shaft is damaged.