The changes in the thermal noise spectrum of a scanning-force-microscope cantilever upon approach of the tip to the sample were used to investigate the interactions between the cantilever and the sample. The investigation of thermal noise is the natural choice for dynamic measurements with little disturbance of the sample. In particular, the small amplitudes involved ensure linear dynamic response. It is possible to discriminate between viscous coupling, elastic coupling and changes in the effective mass. The technique is versatile in terms of substrates and environments. Hydrodynamic long-range interactions depending on the sample, the geometry and the ambient medium are observed. The dependence of hydrodynamic interaction on various parameters such as the viscosity and the density of the medium is described. For sufficiently soft surfaces, the method is sensitive to viscoelastic properties of the surface. For example, the viscous coupling to the surface is strongly increased when the surface is covered with a swollen `polymer brush'.