| I had the same problem also. At first, it was the low action, so I raised it. But what I found out was, it's a combination of problems.
Your neck might not be bent enough (adjust truss rod), your nut might have been grinded down (replace the nut), and finally, your action might be too low. Generally, raising the action is our first response, but it should be the last. Some players have their actions dead low but create no buzz or rattle.
If ALL fails, maybe its a loose electronics part or a bad tuning peg. Play your instrument unplugged and listen to all parts of the bass while you're playing, as in the head, the left side of the string you're fretting, the right side, and the body. It can be a combination of all. Then if you STILL can't figure it out, maybe you should get it checked out at a local shop. Make sure you give them the exact details of the problem, so they wont just over look the problem.
Hope that helped | 
