Under some conditions of power modulation, an rf ICP will exhibit a rapid transition between E and H modes following application of the rf power. It is shown that in noble gases this transition may be connected with the dynamics of the electron density and metastable density of the atoms, and competition between direct and stepwise ionization of the atoms by electron impact. A simple model allows us to demonstrate that after application of rf power the initial slow growth of electron density changes to a rapidly rising function. This rapid rise is consistent with observed E-H transitions that take place in these discharges. The model thus allows us to calculate the characteristic time for the transition beteween modes.