Start unplugging stuff. First, take off/unplug all fans except the cpu fan. Load up the game, if the sound comes back, unplug the audio cable and the monitor while the game is running. If it still happens, try to get a new drive. If the sound happened before the psu/gpu, then the only part left is the motherboard/cpu fan. Replace the cpu fan. Swapping with a case fan works the same, they're all fans. If it still happens, then it's your motherboard. It could be a small fan in your motherboard that's cooling your chipset that's making the sound. You can take it out, as long as you install a fan somewhere to cool it. Alternatively, unplug the motherboard speaker if it has one. You don't really need it. It would just make it difficult to diagnose boot problems, at which point you could just plug it back in later. If anything else was wrong with your motherboard, it wouldn't make that sound. It would more of a popping sound since the only thing that can fail are capacitors which are usually solid and can't make long sounds.