We demonstrate spin-torque induced coherent auto-oscillation in magnetic tunnel junctions of composition PtMn/CoFe/Ru/CoFeB/MgO/CoFeB and of low resistance-area product. At the generation threshold, we observe a strong line narrowing down to 6 MHz at 300 K and a dramatic increase in oscillator power, yielding spectrally pure oscillations with extremely low flicker noise. The induced auto-oscillations are observed even at zero applied field. The frequency of the oscillation mode, and its dependence with easy and hard axis fields are consistent with an acoustical excitation of the two layers of the synthetic ferrimagnet subsystem. Setting the synthetic ferrimagnet into auto-oscillation requires the current polarity that transfers electrons from the synthetic ferrimagnet to the free layer. In auto-oscillation mode, line jitter is observed such that it is the line envelope that is measured in most cases. The line properties for applied fields near the instability boundaries of the Stoner astroid of the free layer indicate that the phase coherence of the auto-oscillation is likely to be affected by the free layer magnetic noise.