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Guided wave structural health monitoring offers the prospect of continuous interrogation of large plate-like structures with a sparse network of permanently attached sensors. Currently, the most common approach is to monitor changes in the received signals by subtraction from a reference signal obtained when the structure was known to be defect-free. In this paper a comparison is made between this defect-free subtraction approach and a technique in which low-frequency vibration modulation of guided wave signals is used to detect nonlinear scatterers. The modulation technique potentially overcomes the need for the defect-free reference measurement as the subtraction is now made between different parts of an externally applied low-frequency vibration. Linear defects were simulated by masses bonded onto a plate and nonlinear scatterers were simulated by loading a similar mass against the plate. The experimental results show that the defect-free subtraction technique performs well in detecting the bonded mass whereas the modulation technique is able to discriminate between the bonded and loaded masses. Furthermore, because the modulation technique does not require a defect-free reference, it is shown to be relatively independent of temperature effects, a significant problem for reference based subtraction techniques.