Thief is pushing the Xbone's limits a little tighter for rendering, so those sounds are from a higher RPM Disc Speed
The game isn't loaded from the disc, it loads from the hard drive. Doesn't matter how much the game is loading, it's all loaded from the hard drive.
When you put in a disc, it does an initial check to see which game it is. If the game's installed, the disc drive turns off and doesn't do anything until you eject. If you're hearing the disc drive after you put an installed game in, you're either hearing something else or your disc drive just doesn't stop when it should, which is unlikely.
That makes more sense, seeing as bf never did it after first putting in the disc. I need to figure out why the disc tray keeps searching... hmm...
Pokemon Y Friend Code: 3454 - 1170 - 2852 Ghost Type with Spiritomb.