Why is there abnormal noise when shifting gears without fully depressing the clutch?

[WalterMarie]

Due to gear grinding. The details are as follows: 1. Meaning: Transmission gear grinding is essentially a hard collision between two metal gears. The result is obvious—accelerated wear on the gear teeth crowns. Over time and with repeated occurrences, the originally right-angled gear teeth crowns become rounded, leading to incomplete engagement after shifting. Even slight vibrations can easily cause gear disengagement, at which point a major transmission overhaul is required. 2. Cause: Transmission gear grinding is caused by operational errors that damage the transmission gears. For manual transmissions, it is generally required to fully depress the clutch before shifting gears. The clutch is then released once the vehicle speed and engine speed are roughly synchronized, completing the shift. Gear grinding often occurs when the clutch is not fully disengaged before shifting, resulting not only in gear noise but also increasing the likelihood of gear grinding. Additionally, if the transmission lubricant contains large impurities, such as iron filings from long-term wear, these can get caught between the gears during rotation, also leading to gear grinding.