Here's why:
* Synchronizers: These components in a manual transmission are responsible for matching the speeds of the gears before they engage. If a synchronizer is worn or damaged, it can't properly match the speeds, leading to grinding or crunching noises when shifting.
* Noise Decreases with Less Throttle: The noise being lessened when you ease off the gas indicates that the gears are closer to being synchronized at lower engine speeds. With less power applied, the difference in speed between the input shaft and the 5th gear is smaller, making engagement easier (though still noisy due to the faulty synchronizer).
* Fifth Gear Specific: The fact that it only happens in 5th gear isolates the problem to that specific gear's synchronizer.
Other less likely possibilities include:
* Worn Clutch: A severely worn clutch could contribute to a similar noise, but it's less likely to be isolated to just 5th gear. Clutch problems often show up more broadly in all gears.
* Transmission Mount: A bad transmission mount could cause the transmission to shift slightly, leading to unusual noises, but it's less likely to correlate so directly with a specific gear.
In short: The most probable cause is a failing 5th gear synchronizer in your transmission. You should have a mechanic inspect your transmission to confirm the diagnosis and discuss repair or replacement options. Ignoring the problem could lead to further transmission damage.
