We analyze cosmic microwave background (CMB) data taking into account the effects of a momentum dependent effective sound speed (MESS) . This approach allows to study the effects of primordial entropy perturbations in a model independent way, and its implementation requires a minimal modification of existing CMB fitting numerical codes developed for single scalar field models. We adopt a phenomenological approach, and study the effects of a local variation of the MESS around the scale where other analysis have shown some deviation from an approximately scale invariant curvature perturbation spectrum. We obtain a substantial improvement of the fit with respect to a model without MESS, showing that primordial entropy perturbations can be an explanation of these deviations.