Waveguide Coherent Anti-Stokes Raman Spectroscopy (CARS) can be used to measure Raman-active vibrations in thin-layer dielectric waveguides. In waveguide CARS experiments, background-free spectra can be obtained when asymmetric mode combinations are applied. The degree of suppression depends on the waveguide parameters and the wavelengths used. A new method using scanning pump and Stokes beams in waveguide CARS experiments is presented, which allows the possibility of maintaining full background suppression conditions over large spectral intervals. A small controlled change in the tuning conditions results in a heterodyning of the signal with a small amount of background, thereby enhancing the small resonant signals. Several simulations for dielectric waveguides are given.