DS218 occasionally beeping for no apparent reason

[blackcat]

Hi,
Today my DS218 is making occasional long beeps (roughly 10 seconds each). I can't find any errors in the DSM. I checked Control Panel - Hardware and Power (it says Reason for current beep: None), Storage Manager, and Notifications. The led lights are green.
Thanks!

 

[blackcat]

OK, I am a total idi0t. The beeping was coming from the UPS, not the NAS.