MS Outlook continuous disk access

Periodically (like right now) MS Outlook goes into a mode where it is continuously accessing the disk. All antivirus is up to date, and there's no real network traffic of any kind; MS Outlook just goes to sleep and the disk access is basically continuous. This will go on for about 25 minutes every 3-5 days or so. I don't see any process taking a lot of resources (right now System Idle is between 97-99% of the total activity). Can anyone suggest what might be going on here? This may be old news to the rest of you, but I'm mainly an end-user here (I do RF for a living).

Thanks,
Dave Maples