According to Alpha_One:
It's typical for manual transmissions to have an unsynchronized reverse. Since the forward gears are synchronized, engaging a forward gear before sticking it into reverse synchronizes the input shaft with the car's speed (i.e. it stops the input shaft) and prevents the grinding noise. "Gently" easing it into reverse also gives the input shaft the opportunity to slow down a little before the transmission slips into reverse.
I had this issue too, from the first test drive. Shifting into first before reverse has eliminated the crunching noise on my car.
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View my fuel log 2014 Mirage DE 1.2 manual: 53.1 mpg (US) ... 22.6 km/L ... 4.4 L/100 km ... 63.8 mpg (Imp)