This happened 4-5 times. I put in Fable 3 and the console was fine but I heard this low pitched beeping sound (not VERY low pitched, but not high pitched either) and occasionally when I played. It happened with Black Ops as well. It did not interrupt gameplay, it just beeped. Not sure if it's the disc drive going on me or what, I've only had the thing since Christmas. I tried putting Battlefield 3 in and it did not beep.
Here is a video I recorded of the sound. You may have to turn up the volume a bit, and it is near the end. You'll probably know when you hear it. It kind of sounds like a check-out at a grocery store.
it's an Xbox 360 SLIM, just so you know. I put in Fable 3 in again a few minutes ago and it was fine. I even played around 10 minutes of the game and didn't get that beep.. thank goodness. EDIT: To be precise the beep is at EXACTLY 2 seconds in.