The pivotal issue in the study of the cuprates is to determine the bosonic glue that leads to superconductivity. In general, a mixing of the phononic and magnetic degrees of freedom is possible. Isotope shifts suggestive of phonon involvement have been observed in photoemission and tunneling measurements of the Bismuth-based compounds. Our recent inelastic neutron scattering work revealed well-defined (in energy) novel magnetic excitations in the pseudogap state in HgBa2CuO4+d (Hg1201) at similar energies. Although the magnetic nature of these modes at relatively low momentum transfers has been established from polarized neutron scattering, our latest results point toward a possible mixing of magnetic and lattice degrees of freedom at large momentum transfers. We propose to perform measurements on 18O isotope-exchanged Hg1201 samples to determine if there is an isotope shift of the mode energies.