It makes one high pitched sound as it starts spinning the disc, then there's a constant hum of it being read. You couldn't mean that, right?
It's just like the high pitched sound as it starts spinning the disc but not quite as loud - it becomes less prominent once the disc is at full speed but it's the same kind of noise ("weeeeee") that continues along with the "hum".
Is it normal to have a constant high pitched noise coming from the disc drive when a disc is spinning?
I uploaded this video where you can hear the noise:
It's most obvious 9 seconds into the clip - you'll need to crank up the volume since the quality is poor. The noise is constant, the variation you hear is simple due to the movement of the camera when trying to capture the noise.