Posted on this earlier and received no responses. Mine does this. A couple of observations:
1. The problem seems to be that when it's supposed to vibrate (including when switching to mute mode or when receiving a message in mute mode) it rings instead. Quite embarrassing in meetings.
2. Reset seems to fix it, but only for up to a couple of hours, then it's back.
3. Butler told me to erase apps & data. Didn't work.
4. Butler told me to Doctor. Didn't work.