rear end noise

To find out the gear ratio is easy. Jack up the back of the car (rear end on jack stands. With the tranny in neutral, you're gonna rotate the tire and count the number of times the driveshaft rotates for one rotation of the tire. Put a piece of tape on the shaft to get an accurate count. For one rotation of the tire -- 2 3/4 rotations of the shaft is a 2.73 rear; a little over 3 turns of the shaft is a 3.08; 3 1/4 turns is a 3.27; 3 1/2 turns is a 3.55; 3 3/4 turns is a 3.73; a little over 4 turns is a 4.10.

As for the noise when unloaded it could be lots of things. First thing I'd check is the condition of the u-joints. When they're shot they can make noise and vibrate in an unloaded state. If the joints are ok - then it's probably time to pull the rear cover and take a look inside. I'd measure the backlash (slack between ring and pinion). If it's out of spec, either the person who set the rear end up didn't know what they were doing, or things (Likely bearings) are beginning to wear back there. If you don't understand any of that -- best to take it to someone that does. Setting up a rear end properly is a bit of tricky business and requires some special tools.

By the way - until you get it fixed, the 'bat outta hell burnouts' are probably not doing your rearend or ujoints any good.

You got bigger tires on there??? My car started makin a grinding noise from the rear end when cruising and i assumed the worst....put it up on a lift at work....all it was was my r/r tire (295/50/15) rubbing the exhaust. Make sure nothing obvious is rubbing before you freak out like I did

Probably a bad axle bearing. If it is that and you continue to drive it, It will mess up the axle. Also, check the wheel bearings as well. That could also be it. I had to replace axles and bearings on both of my stangs. One this week. Good luck and have fun!!