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Hello, when a manual transmission grinds only in one gear, it is caused by a worn syncronizer blocking ring. the blocking ring is what slows down the gear to allow the shift sleeve to engage the gear without making any noise. You would need to remove the transmission and tear it down and replace the 4th gear syncronizer and 3/4 shift sleeve to fix this concern.
Hello, do you need more help or information for this concern?