The proton nuclear spin-lattice relaxation rate 1/T-1, has been measured as a function of temperature (T=1.4-4.2K) and magnetic field (H=0.6-5T) in the Fe8 cluster in its high spin S=10 ground state. The strong temperature and field dependence of 1/T-1, can be fitted well by using a simple model in terms of the thermal fluctuations of the total spin S=10 of the cluster originating from the spin-phonon interactions. 1/T-1, data of protons in a partially deuterated Fe8 cluster is also reported. The results indicate that the spin phonon coupling constant is not affected by the change of mass of the isotopes in the Fe8 cluster.