A 4-speed fully-synchronized transmission making noise only in first and reverse can be attributed to:

The correct answer focuses on the specific characteristics of noise in first and reverse gears in a fully synchronized transmission. In this type of transmission system, noise experienced specifically in first and reverse gears often points toward issues related to the mechanisms engaged in those gears.

Typically, first gear and reverse gear share configurations that are distinct from those involved in the other gears, particularly in their engagement mechanisms. The noise could suggest an issue with the synchronizers or the engagement of the gears themselves—potentially worn or damaged components that are specific to first and reverse gear engagement but not necessarily the main drive or cluster gears directly.

A broken tooth in either the main drive gear or the cluster gear would generally cause noise in multiple gears, not just isolated to first and reverse. Therefore, since the noise is limited to those two gears, it suggests that the source of the problem lies within the components specifically driving those gears rather than the more significant drive elements of the transmission.

Thus, while it is important to rule out both broken main drive gear teeth and broken cluster gear teeth, the situation described is best understood by focusing on the unique characteristics of first and reverse gears rather than immediately assuming a failure in these larger, shared components. This leads to the conclusion that the situation is appropriately categorized