When the car is in gear there's a loud whining sound coming from the gearbox. The sound will stop when the clutch is pressed and the gearbox is disengaged. The sound can increase in volume and pitch as the speed increases.

Gearbox temperature warning light showing on dashboard

There is a light similar to the one pictured here showing up on the dashboard. Different manufacturers sometimes have slightly different warning light symbols but it should be similar to the one pictured.

Gear changes are difficult

Changes between gears have become difficult, the gearstick no longer moves freely between gears, it sometimes seems to get stuck or won't go into a gear properly.

Gearbox makes grinding sound

When changing gear the gearbox makes a loud grinding noise. This will be a more common occurrence on cars with manual gearboxes but can also happen with automatics. There can be various reasons for it but it always presents as a loud, unpleasant sound of mechanical scraping/grinding.