I know this is a 5-month old question but someone might benefit from my answer so here it goes.
I have a 2006 Mazda 3 that had the same problem right after I replaced the rear brake pads but retained the original rotors. After about a week, it developed a humming noise at low speeds, similar to the sound when you rub a wet finger on the rim of a wine glass. At times, it seemed like there were trumpets in back of the car and it can get quite annoying and embarrassing when going out of the parking area. The noise goes away though at speeds above 20 MPH.
After a thorough inspection, I noticed that my rear rotors developed a slight lip at the inner and outer edges. The noise is caused when the edges of the inner brake pads hit the lips of the rotors. The replacement Bendix brand pads that I used had a clip system for the inner pads instead of having springs on top, as with my original Mazda brake pads. Unfortunately, the clips do not do a good job of holding the inner pads well in place thus after some running, the edges of the pads hit the lips of the rotors and the resulting noise.
To fix this, you might need to get OEM style brake pads (same type as your original pads). I got a new set of brake pads (Bosch) that looked the same as the original Mazda pads, ones with the spring on top and the noise went away.
In case replacement with the correct pads still does not work or if the noise comes back, then you need to get new rotors or have your current rotors resurfaced, granted the thickness is still within standards.