When the engine is idling and the gear box selector is in Neutral all the internals of the gearbox are rotating. Pushing in the Clutch disconnects the drive from the engine to the gearbox but it takes a few seconds for the gears in the box to cease rotating. All the forward gears have synchromesh so there will be no crunching noise ifa forward gear is selected immediately the clutch is pushed in before allowing the gears to stop spinning. On the other hand Reverse has No synchros and therefore selecting reverse before waiting a few seconds for the gears to stop spinning will produce the crunching noise you have described.
So it pays to wait a second or two after pushing in clutch before Reverse is selected to minimise the crunch and if the clutch appears to be working correctly in normal driving ,then no adjustment is necessary.
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It sounds like reverse gear is damaged and now there is a problem with selecting reverse.
If the clutch was not disengaging fully then you would be having a problem selecting other gears also and you would be grinding the box on changes. If it only happening in reverse I believe the problem is in the gear box with a damaged reverse gear.
Reverse can be readily damaged from drivers slamming the car into reverse before it has stopped moving forward. If you are not fully stationary before selecting the reverse gear you will damage the box.
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