A few months ago I noticed some reports about such an issue, I think
it's related to systemd v251. Here are some links and a proposed
workaround:

https://bbs.archlinux.org/viewtopic.php?id=276754
https://forum.endeavouros.com/t/solved-loud-beep-at-shutdown/27389/5
https://github.com/systemd/systemd/issues/23520