The info page for grub configuration says in the entry for the option 'GRUB_TERMINAL_OUTPUT': 'Valid terminal names depend on the platform, but may include...**'morse' (Morse-coding using system beeper)** [emphasis mine]
https://sb-bananachips-club.b-cdn.net/media_attachments/files/111/451/916/192/783/412/original/6a7eaf3e11365d2b.png