This one is in the scene where Wil sees Viola for the first time at the party at her house. He sneaks in with the band and plays the tambourine with them. When he sees Viola he stops tapping the tambourine but the sound proceeds for one or two more times, then abruptly stops. It's pretty noticeable too.
Think about this for a moment. If they had a tambourine there, then obviously they must have had someone there to play it. I don't think they would carry around one just in case a friend wanted to come along. And obviously they used one in the band, or else why would they even carry one? So someone must play it, and when William was done pretending, the person who usually plays it probably started to do his job then.