A four-speed transmission is making a grinding noise only while shifting into second gear. What could this be caused by?

The grinding noise when shifting into second gear is often indicative of issues related to the synchronizer mechanism, which is responsible for matching the speeds of the gears during a shift. A faulty 1-2 synchronizer sleeve would mean that the synchronization process is not occurring properly.

The synchronizer sleeve aids in allowing the gear to mesh smoothly by engaging the gears together once they are at matching speeds. If this component is worn or malfunctioning, it can lead to difficulties engaging the gear, resulting in a grinding noise as the gears clash instead of smoothly engaging.

While other factors, such as a worn clutch, incorrect fluid type, or a damaged input shaft, may contribute to transmission issues, the specific complaint of a grinding noise during the transition to second gear strongly points to problems with the 1-2 synchronizer sleeve as the most likely cause.