A remarkable amplitude modulation is observed on a probe beam, when a pump beam is phase modulated in an electromagnetically induced transparency setup in Rb vapor. We observe a phase transition in the probe transmission spectrum by scanning the two photon detuning between the probe and pump for different modulation frequencies. This transition is between the adiabatic regime where the EIT linewidth is broader than the modulation bandwidth and the non adiabatic regime where the EIT linewidth is narrower than the modulation bandwidth. The spectrums of those two regimes are very different where the first exhibits a broad flat EIT spectrum and the latter has a discrete distribution of sharp EIT peaks.  We show experimentally a mapping of this transition in the time domain using a flat top-hat probe pulse. In the adiabatic regime the probe transforms into a series of short pulses having the frequency of the modulation. For the non adiabatic regime an interference pattern is added to these pulses due to the different EIT peak responses overlapping in time and interacting. Applying an external magnetic field on the atomic media results in a Zeeman splitting of the EIT peaks. This splitting creates a more complex interference pattern that resembles multi level Stückelberg oscillations.