Am I imagining this? The day I got it I installed it and from what I remember the game played off the disc as my machine was whisper quiet and the disc did not get hot. Today the machine is pretty loud, the disc is getting pretty hot and the symbol next to the Play Game tab on the dashboard is of a disc and not the HD.

I had a search online and it says that it spins for 5-10 minutes and then stops but I have been on it for around 2 hours now and it hasn't stopped spinning at all

Reinstall it then.

@BleedingStarX: I've noticed this as well. With the game installed, the disc occasionally spins from startup - usually stopping around 20 minutes afterwards at the most. I've not noticed it continuing long after that though.

Only at start, usually by the time I'm ingame its stopped spinning, took me by surprise too.
Think I read its a security check if there's indeed a physical drive in the loader and not some mod? I dunno that though, not up to date on that stuff.

http://www.giantbomb.com/gears-of-war-3/61-30615/installed-game-disc-still-spins/35-515646/