In the diffusion-collision model, the unfolding or backward rates are given by the likelihood of secondary structural cluster dissociation. In this work, we introduce a backward rate calculation modeled from a Kramers-type thermal tunneling through a barrier, which represents the free energy potential well for buried hydrophobic residues. Our results are in good agreement with currently accepted values and the approach suggests a link between the diffusion-collision and folding funnel models of protein folding.