Fusion-evaporation cross sections have been measured for S+" Pd to high accuracy in the energy range around the Coulomb barrier. The two extracted fusion barrier distributions display very similar structures with two well-resolved peaks below the nominal barrier, which are consistent with a model calculation including the coupling of the one-, two-, and three-phonon collective vibrations of " Pd. A third lower-energy barrier, together with a strong relative cross section enhancement, is observed for S+" Pd, and is reproduced by coupling the two-neutron pickup channel which has a large and positive ground state Q value (+5.1 MeV). For S+" Pd, the calculated effect of the corresponding transfer channel (with negative Q value) is much smaller. PACS number(s): 25.70.Jj Many years of experimental and theoretical studies have supplied a lot of information about the dynamics of subbarrier heavy-ion fusion (see [1,2] for recent reviews), but various basic questions about the quantum-mechanical barrier penetration process, in the presence of internal degrees of freedom of the colliding system, still remain unanswered. Essentially, in spite of the good or even excellent results obtained by coupled-channels (CC) calculations in many systems (see, e.g. , [3,4]) both for cross section enhancements and for angular momentum distributions [5], a clear-cut identification of the relevant degrees of freedom involved in the fusion mechanism is often missing. Recently, it has been proposed [6] that the distributions produced by the splitting of the nominal barrier into a manifold of barriers, due to the coupling of channels, can be obtained by accurate measurements of the cross sections by simply differentiating twice the fusion excitation functions. These distributions allow a much deeper insight into the fusion dynamics than the cross sections alone, being more directly linked to the coupled channels. This has triggered a renewed interest for subbarrier fusion studies, and representations of barrier distributions were actually extracted for various systems involving the collision of a spherical nucleus (' 0) with a deformed target (' Sm, W) [7 -9]. The barrier distributions were found to be very sensitive to the nuclear deformation parameters, and the extracted values are in good agreement with those obtained, e.g. , by Coulomb excitation. Such data were also successfully analyzed in terms of CC calculations employing the interacting boson model to describe the structure of the colliding nuclei [10].Characteristic distributions are also predicted [11] for coupling to nucleon transfer channels with positive and negative Q values. More recently, nuclei without stable ground-state deformations were considered and our group performed a high-On leave from Shanghai Institute of Nuclear Research, Shanghai 201800, China. precision study of Ni+ Ni [12]; in this case, a barrier distribution with three clear peaks was observed, which could be explained by double-phonon couplings in both nuclei. This was surprising, since they do not have a ty...