I have a friend with a 84 rx7 and something just messed up on it. He came to a stop light, and when the light turned green, he punched it. He didn't spin the tires but suddenly he heard a really loud grinding sound coming from the rear end. The sound has died down some but it is still there. It now sounds like a supercharger whine. You can only hear it when he is accelerating. What do you think could be a problem?
Thanks

Troubleshooting noises, grinds and the like can be a bitch. Could be a driveshaft flange, differential bearing, rear wheel bearing etc..
Does he have a GSL or GSL-SE model? Both models came equipped with LSD rear-ends.. I would hunch that if the noise just started after a hard launch- check the driveline flange (u-joint) first, then the wheel bearings, then the differential. Here's a nice guide too:

http://www.mazdatrix.com/g2.htm

My friend figured it out. He ran it out of gear oil.

Hmm.. They rarely run dry. Tell him to put redline in there. His rear end will love him for it. Do the same with the tranny (if it's a standard).