Gottlieb player units (which are only on multiplayer pins) result in a fairly loud sound with each player advance. When it’s just a single player on a 4-player machine, it will step three times in rapid succession between each ball, as it moves through the unused spots for players 2-4. Williams multiplayers don’t do that, and I don’t think Bally or Chicago Coin do either.

While this makes Gottlieb multiplayers a bit noisier, I don’t find them to be ridiculously loud. But I concur that a missing back cover can greatly increase the noise, so maybe that’s what you experienced.