Thanks for all the suggestions. Vladimir's caught my attention, because I am using Windows deduplication on 2 volumes of this server, and also initially suspected it could be the culprit, but I haven't found any specific dedup settings that would run every 4 weeks. The Optimization runs daily, and the Garbage Collection and Scrubbing run weekly. I would expect to see the data change every week instead of every 4 weeks. Those do happen to run on Saturdays, which is when the data changes occur. So my instincts are still pointing me to dedup, but I haven't found any technet information or other articles that points to a 28 day event that would be the cause. Are you aware of any that occur monthly?
To answer a few of the other questions: This server has 3 volumes on 3 different vmdk files. (OS, and 2 data drives) There is no antivirus running, I have checked the group policies, and also looked through the task scheduler, but don't see any glaring culprits. We've thought about the personal backup issue, but haven't seen any yet, but need to truly verify with either a comparison or I/O logging. I'll look into one or both of those options and dig some more. Thanks again for the ideas. (I'm still leaning toward dedup)