  We discuss the possible existence a spin singlet excitation with charge
$\pm2$ ($\eta$-mode) originating the $A_{1g}$ Raman resonance in cuprate
superconductors. This $\eta$-mode relates the $d$-wave superconducting singlet
pairing channel to a $d$-wave charge channel. We show that the $\eta$ boson
forms a particle-particle bound state below the $2\Delta$ threshold of the
particle-hole continuum where $\Delta$ is the maximum $d$-wave gap. Within a
generalized random phase approximation and Bethe-Salpether approximation study,
we find that this mode has energies similar to the resonance observed by
Inelastic Neutron Scattering (INS) below the superconducting (SC) coherent peak
at $2\Delta$ in various SC cuprates compounds. We show that it is a very good
candidate for the resonance observed in Raman scattering below the $2\Delta$
peak in the $A_{1g}$ symmetry. Since the $\eta$-mode sits in the $S=0$ channel,
it may be observable via Raman, X -ray or Electron Energy Loss Spectroscopy
probes.
