Has your ONT got a battery backup unit in it? it's like a UPS but just for the ONT and I think Beeping can indicate a fault with the battery backup unit like dead batteries. Honestly the backup is p useless and openreach don't even support them properly anymore so I'd just bypass it if that's the case.
The Battery Back Up’s Openreach used to supply don’t Beep to indicate a low or faulty battery. It would just indicate a problem with a, Fault Light.
