A grinding or crunching sound when you put the car into reverse, and a slight resistance when shifting to reverse are pretty common.
Resistance when Shifting
The idler gear inside the transmission (which is responsible for making reverse actually spin in the opposite direction to the other gears) is straight cut, which can make shifting hard sometimes. Occasionally the teeth on the idler are sufficiently aligned with the teeth of the reverse gear they don't mesh. Shifting into another gear, then back to reverse, may spin the shafts inside just enough that when you shift back to reverse the gears are clear enough to mesh, and you can shift.
Grinding/Crunching when Shifting
The grinding occurs when the gear shafts are moving and the teeth catch each other -- there is no syncro on reverse. Letting the trannsmission sit for a second, with the clutch in, will allow the shafts to stop spinning. You can also shift into another gear, thus using that gear's syncro to stop the shaft, and then shift into reverse; first gear will get a closer rev-match than fifth gear, but either should work.
The lack of synchro on reverse is why you can go from reverse to 1st while rolling, but you can't shift into reverse without coming to a complete stop.
