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Is there any chance you would be able to get an audio clip on your phone or audio device and post the file here so I could hear it?
While driving down the road at a moderate speed do you hear any abnormal rumbling or rotational droning noise? After the car sits for several hours and you get in the vehicle and start to hear the noise can you make the noise change in frequency or get louder or go away if you apply the brakes lightly or hard?
Noises are very difficult to diagnose in this format for obvious reasons without an audio clip. It is possible that there might be something going on with the small E brake shoes that are inside the rear rotors. Or this year was notorious to have the brake pads wear unevenly on the rear and it's possible you're getting close to the spring clips on the brake pads that indicate when they are too low and make squealing noises. Normally though this will change intensity when you hit the brakes. The first thing I would have done is a thorough brake inspection to make sure the brake pads are not too thin. I've also seen weird things like stones even sticks and leaves get jammed between the caliper assembly and scratch were scrape on the rotors until it heats up then the noise goes away.
Okay, I'm at lunch I will look into this when I get back and get back with some thoughts this afternoon after I listen to the audio.
It only makes this noise when going backwards? Does it change if you slowly apply the parking brake if it works and is not frozen?
You said that when you apply the brakes when the noise is occurring that the problem does not go away. My question is if you slightly apply the parking brake if your parking brake is functional and not frozen does the noise change or go way?
Based on the audio sound it is definitely a rotational rubbing noise it sounds like possibly the brake shoes rubbing against the inside of the brake rotors on the rear for the parking brake I have seen some cases where the actual seals themselves on the bearings will make noise and once they heat up the rubber softens and the noise goes away. I've also seen where the actual rubber on the tires will make the same kind of noise where it rubs or creeks on this steel or alloy wheel. To rule that out you can use silicone spray and spray along the edge of the tire where it meets the rim on the inside and outside edges. Try increasing the air pressure in the tires to 35 or 40 psi also. These are about the three most common items that make this exact same noise. Let me how it goes.