There are tutorials all over the net on how to take the 360 apart.
Once that's done unplug the SATA and power cables from the back of the drive and remove it, just lifts out, then there are 4 screws on the bottom of the DVD drive that hold the case of it together. After ya have them out and the thing apart look on the inside of the top half of the case that ya removed, you'll probably see the spindle magnet, looks like a silver cylinder/donut shaped little thing. Pull it off there and you'll see the groove in the spindle motor where it's supposed to be. Use a drop, and I mean only a drop, of super glue in that groove and stick the magnet back in there making sure it's as far down as it will go. Let it sit for an hour or so before putting it all back together, the last thing ya want is the fumes from that crap trapped in the drive getting on the lens. After it's sat for awhile put everything back together and that should do ya.
It should look like this, if it does already the grinding noise is something else. If it is the magnet then it should look like this when you're done fixing it.
