Acoustic coupling with unsteady aluminum particle combustion may lead to self-sustained instabilities with large oscillation amplitudes in a solid rocket motor. A numerical analysis of this phenomenon is carried out for instabilities and at instability limit cycles in a set of generic configurations. It is found that the synchronized combustion oscillations can be split in two different contributions to heat release disturbances driving the instability. In the combustion volume, volumetric heat release rate fluctuations result from the cumulative contribution of burning rate oscillations of each individual aluminum droplet experiencing an oscillating drag. The second contribution to heat release oscillations in the SRM corresponds to the motion of the aluminum combustion zone boundary. In the configurations explored, this contribution may reach up to about 40% of the total heat release rate oscillation in the motor and is shown to depend on the way the end life of aluminum droplets is modeled.