Minecarts produce a "sound", when they appear in the players entity visibility range. You can't here the sound, but you can see it in the number of total sounds in F3.

The sound counter resets to 0 when running the /stopsound command. But once the minecarts disappear and appear again for the player, the sound counter is increasing.

Tested on 1.20.4 with and without Optifine in Singleplayer and Multiplayer (PaperMC).

This won't be a problem for most users, but because I created a PaperMC plugin, which uses "coupled" minecarts as trams and metros, at the depots the maximum number of sounds is reached very fast, so no more sounds can be heard by the player.

Edit: I checked again and it's the minecraft:entity.minecart.riding sound. If this sound is stopped with /stopsound, the minecart will no longer produce a rolling sound, even if the ride starts AFTER the command. So the minecart makes the sound all the time, but just with a volume of zero.

Is it possible to change this, so the sound isn't counted if the volume is zero or change it, so the sound appears if the minecart starts moving and stops if the minecart stops?