This work presents a new multi-frequency inversion method to image shape defects in slowly varying elastic waveguides. Contrary to previous works in this field, we choose to take advantage of the near resonance frequencies of the waveguide, where the elastic problem is known to be ill-conditioned. A phenomenon close to the tunnel effect in quantum mechanics can be observed at these frequencies, and locally resonant modes propagate in the waveguide. These modes are very sensitive to width variations, and measuring their amplitude enables reconstructing the local variations of the waveguide shape with very high sensibility. Given surface wavefield measurements for a range of near resonance frequencies, we provide a stable numerical reconstruction of the width of a slowly varying waveguide and illustrate it on defects like dilation or compression of a waveguide.