The dial-up modem sound comes from the connection handshake used by internet modems over telephone lines, especially common in homes during the 1990s and early 2000s. As broadband replaced dial-up, the noise became a shorthand online for old computers, lag and waiting forever for something to load. It still appears in memes, skits and retro tech videos because the screeching sequence is instantly recognizable, even to many people who only know it from internet culture rather than firsthand use.