Try to locate where the rattle is coming from by listening with a rolled up tube of paper.
Remove the case cover and stick some small bits of foam on the case so that the cover has to compress the foam as you tighten the screws.
While the cover is off, check the CD-ROM and Hard drives for movement. There may be some vibration there if they are not firmly fixed in the drive bays. Try the same approach if that seems to be the root cause of the problem.
Be very careful not to block any ventilation holes. Use several small self-adhesive pads, no bigger than a postage stamp.