The same vibration isolation that helps keep the drive quiet and prevent it from resonating throughout the chassis - is also allowing the spindle to flex from centrifugal force. On most of the drives I've seen lately, there is a plastic "shield" around the top of the laser sled. Most likely you heard that, or the outer edge of the disc rubbing against the housing.
If the disc is not being scratched, then great -- but I'd still avoid moving it.