How can I discover the source of an alert chime on OS X At irregular intervals, my MacBook Pro is emitting a chime. Not the startup chime. More of a brief "alert" type sound.

A few minutes will go by, no chime. Then chime, or several in a row. Then silence for a few minutes, etc.

I clicked notification center when it occurs to see what it's trying to alert me about. There are no notifications listed within the last hour.

I went through everything in notifications control panel and turned off audible alerts.

I turned on "do not disturb."

Power is 100%. Machine is plugged in. Light on power input is green.

Wifi is turned off. Ethernet connection is ok.

Retina MacBook Pro mid 2012.

OS X Sierra 10.12 (16A323).
I figured it out.

It was MS Outlook client.

Outlook ignores OS X notification center. Alert config is in Outlook application preferences.