My MacBook's power brick is making weird noises. When it's plugged in, it gets pretty warm, but so have all of my other power bricks, so I know this is normal. But, it makes really weird hissing and cracking noises, similar to a radio station in a car that's nothing but static. Is this normal? It isn't audible unless I hold it right to my ear. Otherwise, my MacBook is excellent; no mooing, no anomalies. Thanks!