The stock rotors rust really quickly. After it rains, or even after a
really humid period, the surface will rust and it gets scraped off the
first time you apply the brakes, making the grinding noise you're
hearing. If you still hear it after the first stop or two, you may have
a more serious problem.

You don't state what car you have, but the Elantras with rear disks are
known for premature pad wear. Usually, rear pads in a car are good for
100K mile or so, but I had to replace mine at 45K. The pads fit the
caliper so tightly that they had to be driven out with a hammer and
punch! When I installed the new ones, I files the ears on the backing
plates to get a proper fit. Hopefully, that will take care of the problem.

its a 2002 hyundai accent

The most frequent issue with drum brakes is that the moisture gets into the
brake dust that's in the drum.  As long as your brakes don't grab, there's
no hazard in ignoring it.  If the noise bothers you enough, you can pull
the drums and clean out the dust with brake cleaner solvent.