Do you notice it when you are rolling slow? I had a rim spacer loose in my rim and it made a sound like that. To check it, wiggle the wheel back and forth on the bike and you'll hear it . If it does, you have to narrow it down to where it is on your rim and then take your tire off and rim tape and look between the spoke holes and you'll see it. Take a little super glue and squirt it through the spoke hole. Be careful to not get it on the spoke nipples.