We examine substrate-to-film interfacial phonon drag on typical spin Seebeck heterostructures, in particular studying the effect of ferromagnetic magnons on the phonon-electron drag dynamics at the interface. We investigate with high precision the effect of magnons in the Pt|YIG heterostructure by designing a magnon drag thermocouple; a hybrid sample with both a Pt|YIG film and Pt|GGG interface accessible isothermally via a 6 nm Pt film patterned in a rectangular U shape with one arm on the 250 nm YIG film and the other on GGG. We measure the voltage between the isothermal ends of the U, while applying a temperature gradient parallel to the arms and perpendicular to the bottom connection. With a uniform applied temperature gradient, the Pt acts as a differential thermocouple. We conduct temperature-dependent longitudinal thermopower measurements on this sample. Results show that the YIG interface actually decreases the thermopower of the film, implying that magnons impede phonon drag. We repeat the experiment using metals with low spin Hall angles, Ag and Al, in place of Pt. We find that the phonon drag peak in thermopower is killed in samples where the metallic interface is with YIG. We also investigate magneto-thermopower and YIG film thickness dependence. These measurements confirm our findings that magnons impede the phonon-electron drag interaction at the metallic interface in these heterostructures.