After hunting all over CivFanatics for anyone else experiencing this issue, I eventually found this old thread which seems to describe the same thing I'm experiencing. Since that thread is 2 years old (and locked) I started my own. I'm reasonably sure the conclusion in that thread is correct -- that is, that the repeating crunching/crashing sound at the beginning of turns is the sound effect triggered when one city building project finishes and the city is ready for another to begin. Part of my problem (and, I suspect, the other OP's) is that I tend to build a lot of cities and, thus, there's potential for the sound effect to repeat... and repeat... and repeat... and repeat... and repeat... and repeat... and repeat... to the point that I want to shove the computer off the desk. I know I could just turn down the volume of all sound effects in the game settings menu, but I don't want to do that, since I like most of the sound effects (and some, like the you-spotted-a-tribal-village whistle, are very useful). I'm hoping that the effect in question is its own separate file somewhere in one of the Civ 6 folders so that I can replace it with a silent, or at least less-annoying, sound file. If anyone can help me find the right file, and offer guidance on dealing with it if it's in some weird format, I would be most appreciative!