This sounds like a Keyboard Ready beep is occurring. You can disable this feature in BIOS Setup.
During power on or reboot, use the F2 key at the Intel NUC splash screen to enter BIOS Setup (Visual BIOS). Click on Advanced and then Boot and then on the Boot Configuration tab. Then, do the following:
After making any changes, remember to save these changes as you exit from Visual BIOS.
Hope this helps,