Grinding when going from neutral to any other movement position running is an indication that the belt is rotating the transmission input shaft and that happens most often when the one-way bearing/sprag is seizing..which is very common on all with them in the primary clutches. Just remove the belt, reach in and spin it with your finger. It should rotate one way smoothly and quietly and lock the other way. If this isn't the condition just replace it. If it is, then look to dirt and dust packing into the flyweights. Clean and service both clutches..which should be done every so often anyway.