The motion of interstitial H and D in Nb observed as diffusion, neutron scattering, ultrasonic damping and internal friction gives hop rates that differ greatly because the tunnelling rates depend very strongly upon the asymmetry in energy between the interstitial wells. Large trap energies of about 0.1 eV mean slow motion between traps for internal friction below 100 K. We extrapolate the tunnelling formulas to large asymmetries and consider the effects of resonance between the excited states in traps and ground states in neighboring regular wells, and find agreement with measurements of internal friction.