mine used to do the same, i got mass effect repaird twice because of it, i then took the xbox appart and opened up the dvd drive, i founda tiny bit of pcb prtruding from the lense i sanded it back with some 2000 grit and it never scrached a disc again.
it did rrod though and ms replaced both the mobo and dvd drive
it did rrod though and ms replaced both the mobo and dvd drive