I have a '10 JKU Rubicon with 47k miles. For a bit now I have been noticing a grinding coming from the back at slow speeds, under 30-35 mph. Once I clear 35 it's gone. I can also hear it only if the windows are down. It will feel like a grinding through the brake pedal as well, but just barely noticeable. If I'm going around 30, with the grinding present, said grinding disappears with brake pressure until I get to 15ish mph.

Any ideas?

 

I would check the condition of the rear brake pads. It's not unheard of that our jeep brakes in rear need to be replaced before 50k. Also, my rear brake pads were rusted like crazy on my 2012 JKU. I would start with the basics. Just replace your rear brake pads and see what happens to the noise. In the process, you will need to compress the caliper piston, so you may see if that is getting hung up.
Look at how horrible my brakes looked on my 2012. So much rust. Pad can eventually disbond from metal backing.

 

Nevermind problem solved. Took the rear wheel off, pad and rotors shot.

:(