• Before you call CSS, I would check the underlying SAN LUNs and be sure there isn't contention there. The SAN admins often look at averages, but if they've carved up physical disks and shared them among LUNs, there can be some contention from other systems.

    No A/V is good, have you also checked if some type of backup or snapshotting was occurring at this time? Anything that might have frozen disk access briefly for SQL Server?