Simon -
That 'No Space' alarm may be the culprit. I just checked two different units here - one of them had that alarm disabled - the other had 'Buzzer' turned off and only made a log entry when space went to less than 20%.
Try turning off 'Buzzer' on that alarm and see if that affects it.
The 'Auto delete old files' is set to 'Never' by default - it will not keep the unit from deleting old files to create space for new files.
(It is a 'Reboot' function to purge files on restart)
If you have 21 TB of storage - you have multiple hard drives - go to Main menu > Storage (in Settings row) > HDD Detect (on left side)
Run a quick 'Detect' on each drive and see if there are any errors.
And lastly - check uner Main menu > Event (in Settings row) > Detect (on left menu) Check the 'Motion' tab and make sure 'Buzzer' is not enabled for any camera. If 'Anti-dither' is set to 5 seconds, and the 'Buzzer' is enabled for Motion - you'll get an annoying beep every 5 seconds. (anti -dither is the time after a motion alarm, before the unit can send another motion alarm such as the buzzer = so if you have constant motion, it would beep every 5 seconds or so)
If none of that helps - then it may be time to look at a 'Factory Reset' to default everything. If that stops the beeping then there was a setting we missed somewhere.
The deciding factor for me on firmware update is - has the NVR alwayws done this or did it just start happening recently?
If it has done this from day 1 - then a firmware update may be the best solution. If it just started recently, it is more likely one of two things = a settings mistake, or potential hardware issue. (hardware issue would probably show something in the logs though)
Ted