I would go for a normal start-up sound for external drive
May have always done it and you did not notice especially if you normally start in different order so hiding this sound
Only guessing without hearing it live but expect it’s just start-up electricity sounds of tension
Just as a double check test the disk, cannot remember enough on lubuntu but in control panel usually is a disk option and that gives a idea on disk status.
https://help.ubuntu.com/stable/ubuntu-help/disk-check.html.en
Or go to terminal
Type disks
That should run smart (Self-Monitoring, Analysis, and Reporting Technology)
Then check the external disk