You could check if the disk activity is tied to an On Demand Scan that is running, in which case it's expected to see.
The ODS runs from a process named Scan32 or Scan64, and it utilizes the scan engine capabilities that have already been loaded by the On Access Scanner instance. But if the disk I/O is impacting something else that's running, you may need to reconsider the ODS configuration or time at which it runs.
If the disk activity is not from an On Demand Scan running, then you'd definitely want to take a closer look at while file(s) the On Access Scanner is stuck on during that time. The OAS only does work in response to another process touching files, so you'd want to find out what the file is (or files if there are multiple), and find out which process initiated the activity that caused us to intervene and do work. A config change will resolve the issue.
Whatever the case, Support could assist you further in lieu of more meaningful details.