Brake Disc Rotor problems of the 1997 Infiniti I30

Rear brake hum at low speed, mainly when accelerating from full stop. Both rear rotors replaced and pads replaced, still occurs. Occasional grinding from driver's rear brake - mechanic cannot solve problem despite frequent brake part replacements. Dealer cannot find problem either.

Took the brake rotors to be cut, the technician told me that the rotors were cracked.