In the LT-4 tokamak, the precursor activity for disruption at q(a)≤2.5 does not exhibit the oscillatory low order mode MHD behaviour usually seen elsewhere. Instead, each thermal quench is preceded by a fast transient perturbation with dominant Fourier components m/n = 2/1 and 3/2. The corresponding islands grow so rapidly, at rates consistent with the Rutherford non-linear tearing mode theory, that they reach estimated sizes sufficient to cause disruption in less than a typical Mirnov oscillation period.