The interaction between two whistler waves with amplitudes B and &egr;B and frequencies &ohgr;r and &ohgr;r+Δ&ohgr; creates a third mode with frequency &ohgr;r-Δ&ohgr; whose amplitude might significantly exceed the level of the weakest interacting wave. The mechanism relies on the nonlinear dependence on B of the large whistler mode propagating characteristics and is unrelated to synchronous wave coupling.