In structural health monitoring, it is often necessary to make reference measurements on a baseline healthy sample. However, baseline parameters can be strongly affected by environmental and other conditions and significantly change over time. Here we propose a baseline-free damage localization method consisting of a repetitive pump-probe experiment in a metal plate and implementation of a back-propagation algorithm. In the reported experiment, a PZT patch generates a high-frequency (HF) probe wave at 20 kHz, while a continuous low-frequency (LF) pump of 1 Hz is produced by a shaker. Signals propagated in the plate are recorded using a set of PZT sensors glued at known positions. We use a steel ball pressed against the plate to mimic a nonlinear defect. The aim here is to produce a solid-solid contact that will be modulated by the LF pump, as would be the case, for instance, in fatigue cracks. To enhance this effect, we subtract signals recorded at different times (corresponding to different loading states) and apply the back-propagation algorithm that locates the origin of the modulation. The procedure is repeated several times to enhance the localization quality. In future, we plan to use the method with a real defect, thus exploiting more realistic contact acoustic nonlinearity.