  MuSR experiments have been performed on powder sample of the "ordered spin
ice" Tb$_2$Sn$_2$O$_7$ pyrochlore compound. At base temperature (T=35mK) the
muon relaxation is found to be of dynamical nature which demonstrates that
strong fluctuations persist below the ferromagnetic transition (T_C=0.87K).
Hints of long range order appear as oscillations of the muon polarization when
an external field is applied and also as a hysteretic behavior below T_C. We
propose a dynamical and strongly correlated scenario where dynamics results
from fluctuation of large spin clusters with the "ordered spin ice" structure.
