Yeah it's because of the Lua engine for private classic servers doesn't have any easy way to do some of the checks on abilities, so the sound you hear is the bot opening the spell book picking up a spell making a check then removing the spell again and closing the spellbook. It just goes so fast that you don't see it.
Unfortunately that is not something I can make any workarounds for, so my only suggestion is to turn off your sound while botting. 🙂
