In low-temperature resonant Raman experiments on MoSe2-WSe2 heterobilayers, we identify a hybrid interlayer shear mode (HSM) with an energy close to the interlayer shear mode (SM) of the heterobilayers, but with a much broader, asymmetric lineshape. The HSM shows a pronounced resonance with the intralayer hybrid trions (HX−) of the MoSe2 and WSe2 layers only. No resonance with the neutral intralayer excitons is found. First-principles calculations reveal a strong coupling of Q-valley states, which are delocalized over both layers and participate in the HX− with the SM. This emerging trion-phonon coupling may be relevant for experiments on gate-controlled heterobilayers.