Typically, the dynamic behaviour of magnetic nanoparticles is investigated by measuring their response to externally applied magnetic fields. In contrast, we present a study of the magnetic fluctuations in an ensemble of magnetic nanoparticles recorded in the absence of any external excitation. Several samples of magnetic nanoparticles with varying particle size, composition, and environment were investigated. We interpret the thermal magnetic noise spectrum to estimate particle size distributions and compare these to the distributions derived from magnetorelaxometry measurements of the same samples.