When driving along the gear stick will suddenly pop out of gear into neutral, this will often result in over-revving the engine if your foot is on the accelerator. It will often also be difficult to change into some gears.
When the car is in gear and driving along there's a screeching and/or a creaking metallic sound coming from under the centre of the car. The sound will change and get worse as the car moves and accelerates.
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.