Why Transmission Produced Grinding Sound When Shifting in One Particular Forward Gear

The reason for transmission grinding sound when shifting in forwarding gear

(1) The faulty synchronizer of gear which is grinding

As indicated in the problem, grinding only occurs in one forward gear. This generally indicates a problem with the synchronizer of that particular gear. The synchronizer is a device inside the transmission which brings the rotating shafts/gears together at the same speed for smooth shifting. If it is faulty, grinding will generally result and, in extreme cases, it will be impossible to shift into that particular gear.

Incidentally, if the gear in question is first gear and the grinding only occurs when shifting into first from a stop (and not when downshifting into first), it's also possible there is too much clutch pedal "free play". Free play is the distance the clutch pedal moves before the clutch begins to disengage (1" - 2" is normal). In this situation, even when the clutch pedal is down, the clutch isn't fully disengaged and thus when shifting into first gear, grinding results.

What to do?

If this vehicle is under warranty or a service contract, you're covered. Otherwise, you'll need to find a reputable garage that specializes in manual transmissions. Take this vehicle to this garage for an inspection. As was mentioned in the explanation, if the gear in question is first gear, have the clutch pedal free play adjusted as a first step. Never agree to a transmission teardown until you have received several opinions from different shops.