Why does my iOS device beep at noon every day?

I have an iPod touch (4th gen). A few weeks ago, it started making an alert sound (tritone) every day at exactly 12 noon. There is no on-screen alert, nothing in the Notification Center pull-down. Just a sound. Every day.

I have NO apps in my Notifications (Settings). I have NO alarms set. NO reoccurring calendar items.

Anyone have a clue why my device is doing this?

Under the app in the list, there should be a description of which types of notification an app can send. Look through the list for apps that say 'Alerts' or 'Sounds' and disable the 'Alert Tone' or 'Sounds' respectively.