Could be that the starter isn't fully engaging and it has stripped a few teeth off the ring gear and when the starter tries to turn the engine over at this spot on the ring gear you'll hear this loud grinding noise. When the ring gear and the starter gear aren't lined up on the stripped spot, the engine turns over normally. You don't want it to have damaged the ring gear, this can be somewhat expensive.

Good Luck,
Michael
Melbourne, Fla

1985 740 460,000 mi (original motor)
1988 780 Coupe V6 180,000 mi
1992 745 185,000 mi