Coil whine is caused by high levels of stress on the graphics card, often caused when a game menu is rendering at ridiculous fps (>1000). I find that when I enter and minecraft menu that is not in-game or the main menu, I can hear a high pitched noise coming from my computer, which I discovered was coil whine. Now this would be expected if I had minecraft set to max fps without v-sync, but even with v-sync the fps on these menus is not being reduced and is causing the gpu to whine. V-Sync should be applied to the menus and the actual game, not just the game.
When I installed optifine, the problem was fixed, and I only heard the noise when I DISABLED V-SYNC. So I think the problem is very clear.