FOUND THE ISSUE: Seems it's not a known bug and I've reported it on their Uservoice. This seems to happen only on SSMS 18.3.1 onwards up to SSMS 18.5. On SSMS 18.2 and lower the issue does not happen. Thanks everyone!
Each time I use the table designer and the focus changes from any field (Column Name, Data Type, Allow Nulls, etc.) either by pressing the Tab key or switching to another window, the classic windows beep sound plays. And it's annoying as heck, whoever decided it was a good idea should revisit his objective in life.
SSMS (SQL Management Studio) doesn't even appear in the volume mixer, so I can't even mute it from there unless I mute the entire output device (or System sounds) which isn't what I want. Is anybody else experiencing this or is it happening only on my computer?
Typing "beep" in the SSMS preferences doesn't help much either...
EDIT: Thanks for the Autoruns and Windows Sounds suggestion. Unfortunately, even though I disabled all items in there, and SSMS does not show any sound to be played in the Windows Sounds panel, the beep is still there! :(