Sounds to me like your rear brake drums need cleaning, and adjustment of the parking brake may be needed. You may have low brake shoes that need to be replaced and/or drums resurfaced. That could address your squeaking when backing up. For the humming I can not relate to a low speed in reverse, other than a normal rear axle and bearing feedback. It's hard to tell without looking at the vehicle.
You're going to have to do a little detective work to help isolate the noise you're hearing. First of all, how long has your vehicle been making this noise? If it has been doing it the entire life of the car, then it is probably normal, (the way reverse gears are cut causes a whining noise when you back up). Also, your vehicle is probably 4 wheel drive, so there is a chance it could be coming from the rear differential. Has it been a while since you had your rear differential fluid replaced? Also, how long has it been since your transmission fluid has been replaced? Check your Owner's manual to see if you're overdue. The condition of your transmission fluid can often times give you some insight to how your transmission is doing. Dark fluid could indicate over-heating and finding metal shavings in it tells you that you've got a definite problem! Depending on the year and mileage of your vehicle, it could still be under warranty. My advice is to take it to a GMC dealer and have them road test it with you, (they usually won't charge you for this). Also a heads up, there is a chance that this noise could be related to something else, (suspension, tires, even rear brakes!). I'm sorry I can't give you a better diagnosis, but noises are awfully tricky.
Best of luck,
〉 Answered on Feb 27th, 2008 by Jennifer Frey, Automotive Service Writer and Technician at AskPatty.com
Without hearing the noise, it's really hard to pinpoint where the noise is coming from. I would suggest you bring your car in to a technician that can properly diagnose the problem. If you're having trouble finding a technician you can trust, AskPatty has a list of quality and female friendly shops.
〉 Answered on Feb 27th, 2008 by Suzanne Grego, Technician at City of Philadelphia Fleet Management