Which issue is likely if a manual transmission makes noise primarily in neutral?

When a manual transmission makes noise primarily in neutral, it often indicates an issue with the internal components that are still in motion even when the vehicle is not engaged in gear. The main shaft bearing, being a critical component that supports the main shaft during its rotation, can wear over time due to continuous operation and lubrication breakdown. When the bearing is worn, it may not maintain proper alignment or support for the rotating shaft, leading to noise as it can create movement that produces sound without the resistance experienced while in gear.

Worn input shaft bearings and synchronizer rings typically manifest noise when the transmission is under load or while shifting gears, since these components engage actively during those operations. A loose transmission mount may cause vibrations or clunks when the vehicle is moving or under acceleration, but it would not primarily produce noise when the vehicle is in neutral, as the mount itself is more related to the structure securing the transmission to the vehicle frame.

Thus, the most likely cause of noise when the transmission is in neutral points toward a worn main shaft bearing, emphasizing the importance of properly diagnosing bearing issues in manual transmissions.