I hear a grinding(metal on metal) sound as soon as I take off and it's coming from my back passenger wheel. I replaced the breaks less than 3 months ago so I'm pretty sure it's not them. I'm a green mechanic so any help would be appreciated! Thanks!

 

You replaced the rear pads I'm assuming? How did the parking brake shoes look? The backing plates are known to rust out on these, and when they do they could lean up against the disc or drop chunks down into the drum, that may be what is making the noise.