While saving games I notice a momentary coil whine type of sound comes from my computer for a fraction of a second. This type of sound I have only heard while running graphical programs such as 3d games that render at unrestricted framerates which seems to be GPU generated. I know of no CPU/disk/memory load that causes such a sound from this computer over the many years I have owned it. Trying to have the game report FPS seems to indicate a drop but it also stops refreshing during the save. Other ideas I had was I can see if a larger map or more complicated game board takes longer to save to hear it longer and try to see if I can launch the game under more basic VESA drivers to check for reproducibility. Current OS is FreeBSD though I doubt that is a primary factor in this.
This, and maybe my previous message, might be better organized in the bug report area. My apologies if I am placing any posts into incorrect areas.
Simutrans uses multithreading for the compression of the game during saving. It is saving into a buffer while compressing in a second thread. This maximizes CPU load and, depending on the library, may even involve the cryptographic GPU of modern processors or using AVX code inside those libraries.