What is the problem in a manual transmission that is noisy under load in all reduction gears?

When a manual transmission exhibits noise under load in all reduction gears, this is often indicative of wear in the cluster shaft bearings. The cluster shaft is a critical component that supports the gears within the transmission and allows for the proper meshing of those gears during operation.

When the bearings become worn, they can no longer maintain the necessary alignment and support for the cluster shaft. This misalignment can result in excessive movement and increased friction, which generates noise when the transmission is under load. The noise is a symptom of the compromised integrity of the bearings, affecting overall transmission performance.

In contrast, worn synchronizer rings primarily affect gear changes and would not typically produce noise under load across all gears. A damaged input shaft can create problems but often presents symptoms that are localized or may affect specific gears rather than all of them. Low transmission fluid levels can lead to internal issues as well, such as overheating or insufficient lubrication, but it does not specifically cause noise in all reduction gears. Thus, the wear of the cluster shaft bearings is the most logical explanation for the noisy operation across all gears under load.