Chirping observed in ion cyclotron emission (ICE) from the KSTAR tokamak at sequential proton harmonics in the range 200 to 500 MHz has recently been interpreted (B. Chapman et al., Nucl. Fusion 57, 124004 (2017)) as due to fast, sub-microsecond, evolution of the local deuterium plasma density. This density evolution changes the plasma environment of the 3 MeV fusion-born protons which drive the ICE through collective relaxation by the magnetoacoustic cyclotron instability, resulting in fast evolution of the spectral distribution of energy in the excited fields. Here we examine a separate, fainter (“ghost”) chirping ICE feature observed in the higher frequency range 500 to 900 MHZ, which is time-shifted with respect to the lower-frequency feature. We show that it is driven by nonlinear wave coupling between different neighbouring cyclotron harmonic peaks in the main ICE feature. This is evident from bispectral analysis of: first, the measured KSTAR fields, where we benefit from exceptionally high (up to 20 GS per second) sampling rates; and second, field amplitudes output from first principles particle-in-cell code simulations of the KSTAR fusion-born proton relaxation scenario. This reinforces the MCI interpretation of chirping proton ICE in KSTAR, while providing a novel demonstration of nonlinear wave coupling on very fast timescales in a tokamak plasma. The successful interpretation of unexpected phenomenon, which is spontaneously driven by fusion-born ions, helps to establish interpretive capability for future deuterium-tritium plasmas in JET and ITER.