The time-dependent velocity fluctuations exhibited by diffusing molecules in the liquid state cause additional signal losses in Pulsed Gradient Nuclear Magnetic Resonance (PGNMR). While theory suggests that these losses are generally related to the spectral density of such fluctuations, experimental evidence to substantiate this has been lacking. In this study, the spectral density is established as a new NMR measurable and a new contrast mechanism in NMR diffusion imaging. For routine measurements of the spectral density, a PGNMR probe was designed, constructed, and tested. Using the developed probe and specially designed pulse sequences, PGNMR diffusion measurements were performed on water saturated random packings of monodisperse glass spheres. Analysis of acquired density spectra indicates that the precise distribution of low frequency (0-500 Hz) fluctuation modes is governed by the contact collisions that occur between the liquid molecules and the glass spheres. Measurements performed using spheres of different diameter (200-300μm) confirm that the frequency dependence of the spectral density depends on both the bulk properties of the saturating fluid and the topology of the random sphere pack. Analysis of the spectral density is shown to provide information about the bulk diffusivity of the saturating fluid, the tortuosity of the saturated medium, and the surface-to-pore volume ratio of the fluid filled pore space.