Electrical short? There is frequently a high pitched squeal when the gas is pressed. Sometimes, when the radio is OFF the rear speakers turn on. (Only the rear and very low. It took a whole to realize what was happening.) Sometimes when the radio is on and the blinker is used, neither make sound. I worry that there is something terribly wrong, my spouse is afraid if we take the vehicle to the dealer they will charge a boatload of $$ and fix nothing. Any ideas of what this problem is? Also, I can hear loud noises when I turn the steering wheel.

It sounds like your serpentine belt is going. I would have that checked out for wear and also have the pulleys that coencide with it checked as well.

when the radio is OFF sound comes out of the rear speakers when the blinkers are used ! There isn't a turn signal flasher on your vehicle ,( like most all newer vehicles ) It's done electroniclly by a control module . In the case of your vehicle , the BCM - body control module . It doesn't matter if the radio is off . All the Audible warnings come out of the radio speakers . Headlamps left on , key in ignition etc... You have no idea of the complexity of your vehicle , it's a rolling computer , with a serial data network for the modules to talk to each other , share info.
When the turn signal switch is placed in either the left or right position, a ground signal is completed from the turn signal switch to the BCM. When the hazard flasher switch is activated, a ground signal is completed from the turn signal/hazard switch to the BCM. The BCM then sends an ON-OFF voltage signal through the fuses to the appropriate turn signal and side repeater (export only) lamps or to all of the turn signal fuses for hazard operation.
Really don't think you have electrical short . Computer glitch maybe , radio problem . How ever , these system's are not for the untrained to mess with . Your best bet , the GMC dealer !

Most likely a belt slipping!

In my opinion, it's almost certainly the radio. Even when turned off, it is still alive managing a few other things (like chimes) until you exit the vehicle, and I suspect that part of the radio is faulty.

You need to have the brakes inspected. This noise is the brake pad wear warning tab hitting the brake rotor.;

this vehicle will require a new cd/radio. the mother board that controls the cd/radio is burnt out

To see if the noise is coming from one of the accessories driven by the serpentine belt, remove the belt and run the engine for a short time. If the noise is gone, spin each one the pulleys and see if you feel any roughness or the pulley is binding. The belt tensioner pulley and the idler pulley are the most frequently ones that cause squealing noise.

loose fan belt?

Put the car in neutral, set the parking brake, open the hood, get someone to listen/see where the sound is coming from, as you press on the accelerator.

Hi,welcome to fix ya. if this is a bose system you have a bad amp behind the rear seat they short out inside and cause the speakers to emit a high pich squeel. you can buy these amps on e-bay pretty cheaply or send it to bose for repair .

Your rear brake pads are most likely worn to the wear indicator which can make a high pitch squealing noise. It is very common that it will squeal when backing up, but not when applying the brakes. 50,000 miles is about the average life for rear brake pads on your car.