This study presents results of an experimental analysis of the unsteady features of the flow around the rear part of an Ahmed body with rear slant angle of 25o. This analysis focuses on the 3D separated and reattaching zone that develops on the rear slanted surface and brings new information, improving the understanding of the flow unsteadiness. Flow investigations are performed using PIV, hot wire anenometry and unsteady flush mounted pressure transducers in the plane of symmetry above the rear slanted surface. Spectral analysis and Proper Orthogonal Decomposition of the output signals show the emergence of low frequency unsteadiness and high frequencies activities. Characteristic time scales of both instabilities are provided and the physical effect of the low frequency unsteadiness is related to a flapping motion of the separated shear layer, while the high frequency activity is associated to a large scale vortex emission. The results focus in the centerline plane of the rear slant and more specifically demonstrate relevant similarities with the unsteady mechanisms of 2D recirculated flow.