If a howling sound occurs when shifting gears, the tone of which changes as the "gas" is increased or decreased, the cause of this phenomenon may be a worn toothed connection of the corresponding pair of gears.
If there is noise in all gears, the cause is the main gear or gearbox shaft bearings.
A grinding noise that occurs only after the gearbox reaches operating temperature indicates a beating of the synchronizer locking rings. As soon as the oil becomes more fluid, it breaks off the synchronizer ring all the time in the same area.
Hint: workshops, as a rule, rarely decide to repair or overhaul the gearbox, but advise replacing the faulty box with a new one. In financial terms, however, in most cases it is more expedient to install a used unit purchased from a car dismantling site. In this case, you need to pay attention to the correct letter marking of the gearbox. It is stamped on the gearbox at the top, in front, on the left (i.e. on the clutch housing). In addition, the day, month and year of manufacture are indicated there, with the year indicated only by its last digit.
(The text is based on materials from the website «audimanual.ru»)