At low temperatures the acoustic properties of glasses differ from those of crystals. The anomalies can be explained by the existence of localized low-energy excitations with a broad distribution of energy and relaxation times. The dynamics of the excitations, also described as tunneling states (TS) is, on a phenomenological level, well understood for silica-like glasses. The presence of TS in overconstrained amorphous solids like pure bulk Si is however not so well established. We started a low-temperature ultrasonic study of Si, partly amorphised by neutron irradiation. We measured the ultrasonic attenuation (for 320 and 425 MHz) as a function of temperature (0.3 to 80 K) before inrradiation. The data show that the anharmonic phonon-phonon interactions dominate the behavior.