A little help please

I was riding a couple days ago coming down a hill (not steep at all) and pulled over for a second. I put the bike in nuetral and let the clutch out. Then I start to here a little rattling. I start looking around and it sounded like it was coming from the left side by the shifter, I assume the trans. It sounded like little pebbles swirling around, not loud at all. I put the bike in 1st tried to take off, went a few feet and the bike shut off. Started the bike tried to put it in nuetral but it didn't want to go. This happened a few time and then I took of, and it was fine all the way home. Anyone have any ideas?