The reason it dont crunch when it idles realy low is cos the engine is turning very slow. I'E

If you hold you foot on the throttle and hold it at 3-4k revs then most cars will gring as you put in reverse .

If you put a car in reverse when the engine is almost dying then it wont crunch .

Its all to do with the syncro .

Try turning the tv over while wanking as fast as you can, then try while doing it slowly.