In the ADITYA-U tokamak, modulation in the Hard x-ray (HXR) emission from the limiter synchronous with a m/n = 2/1 tearing mode is observed during the flat-top and pre-disruption phases of a discharge with a significant runaway electron (RE) population. The oscillation in the HXR signal is observed for a minimum magnitude of a poloidal magnetic field perturbation , induced by a m/n = 2/1 magnetic island. Moreover, m/n = 2/1 tearing mode harmonics are pronouncedly visible in the HXR signal. The measured HXR energy spectrum in the presence of a 2/1 tearing mode exhibits high counting statistics of HXR photons in the energy range of 1–3 MeV. In contrast, in the absence of a 2/1 tearing mode, relatively low counting statistics of HXR photons are recorded in the same energy range. The modulation of the HXR signal in the presence of the m/n = 2/1 magnetic island is explained by a periodic RE loss due to resonance between the RE drift orbit and the m/n = 2/1 magnetic island radial magnetic field perturbation.