Persistent rapid up and down frequency chirping modes with a toroidal mode number of zero (n = 0) have been observed in the JET tokamak when energetic ions, with a mean energy 500keV, were created by high field side ion cyclotron resonance frequency heating. This heating method enables the formation of an energetically inverted ion distribution function that allows ions to spontaneously excite the observed instability, identified as a global geodesic acoustic mode. It is interpreted that phase space structures form and interact with the fluid zonal flow to produce the pronounced frequency chirping.