Rotors are succeptible to flex, yes. Tire tread, terrain, rotor pattern all effect resonance. For instance, from 15-16.5 mph my rotors make a racket on the pads. above and below the range and its fine. Change to smoother tread or softer ground, and it either changes the speed or goes away.
Keep in mind the pad clearances are very small, so the least ammount of chatter can make an effect. This is seeminly amplified in aluminum(I have both in hardtails). Aluminum resonates sharper and crisper than steel, transferring the energy to anywhere it can dissipate it, which would end up as the rotors. When this is the case, it really is not something to be concerned about. The 'flex' associated with it is not something that is simply fixed, nor is it detrimental.