The fluctuations of the position of monatomic steps on Ag(111) are investigated by scanning tunneling microscopy (STM). We analyze the influence of tip¿sample interaction by varying the gap impedance over more than two orders of magnitude. For tunneling tips providing a weak tip¿sample interaction, we show that the step position autocorrelation function remains essentially unaltered. In this unperturbed case, the kinetics of step fluctuations are found to be dominated by one-dimensional mass transport. For larger variations of the tip sample distance or for less favorable tip configurations, we observe a tip-induced increase of the step fluctuations. Our measurements suggest that this effect is caused rather by short-range forces than by the electric field in the tunneling gap.