Motorola Xoom :: Mysterious Ringtone - No Notification Or Trace Of Any App

In the last two weeks, something is causing my 4G/LTE Xoom to ring like a phone, but there is no notification and no trace of any app running that would cause such a ringing. I have System Monitor installed, and tried closing/stopping processes one by one, but I could not identify the cause. When I stop ALL applications/processes at once, the ringing stops. In fact, besides rebooting, this is the only way to stop the ringing. I have tried looking at running processes/apps when it occurs, but no app with a ringtone like this is apparent. I know it is a rogue application, but I cannot isolate or identify it Lookout and McAfee both show no issues when Xoom is scanned.