The paper reports combined experimental and numerical investigations of unforced un- steadiness in a vaneless radial diffuser. Experimental data were obtained within the diffuser using stereoscopic time resolved Particle Image Velocimetry (PIV) recording three velocity components in a plane (2D/3C), coupled with unsteady pressure transducers. To characterize the inception and the evolution of the unsteady phenomena, spectral analyses of the pressure signals were carried out both in frequency and time-frequency domains and the PIV results were post processed by an original averaging method. Two partial flow rates were investigated in detail in this paper. A single unforced unsteadiness was identified for the lowest flow rate, whereas, two competitive intermittent modes were recognized for the higher mass flow. Numerical analyses were carried out on the same pump by the commercial code CFX. All the computations were performed using the unsteady transient model and the turbulence was modelled by the Scale-Adaptive Simulation (SAS) model. Numerical pressure signals were compared with the experimental data to verify the development of the same pressure fluctua- tions.