Why might a transmission make noise in certain gears while being quiet in others?

The presence of noise in certain gears while being quiet in others often indicates that there may be issues specific to the gears within the transmission system. Worn gears can impact how the teeth of the gears mesh when engaged, leading to vibrations and noise. This deterioration might only manifest in particular gear selections due to variations in the load placed on each gear and its alignment when engaged. For example, under specific conditions or load scenarios, wearing may become more pronounced, resulting in audible noise.

The other mentioned factors, such as sticking synchronizer sleeves, excessive fluid temperature, and incorrect gear oil, tend to affect the overall performance and smoothness of gear changes rather than producing a distinct noise in specific gears. Sticking synchronizer sleeves may lead to difficulty engaging the gears but wouldn't necessarily produce a noise indicative of gear wear. Similarly, excessive fluid temperature and incorrect gear oil can create overall system inefficiencies, but these issues typically do not correlate with noise localized to particular gears. Therefore, the specific focus on worn gears aligns accurately with the phenomenon of noise being present in certain gears and absent in others.