Presented here is an analysis of JET data [1,2] that contains persistent, chirping n = 0 modes, where n is the toroidal mode number. These chirping modes are only found to be experimentally established when there is High Field Side (HFS) Ion Cyclotron Resonance Frequency (ICRF) heating applied and where the power applied by neutral beam injection is not too large (typically <5MW). The issues addressed are the identification and mechanism of excitation of the basic plasma mode being observed. It will be shown that the mode is related to the geodesic acoustic wave [3]and the source of instability must come from the inversion of the energetic ion population, where ðF(E,µ,Pf)/ ðE > 0 (E, µ and Pf are the energy, magnetic moment and canonical angular momentum of particles in an equilibrium distribution F).