  We have studied the origin of excess noise in superconducting transition-edge
sensors (TES) with several different detector designs. We show that most of the
observed noise and complex impedance features can be explained by a thermal
model consisting of three bodies. We suggest that one of the thermal blocks and
the corresponding thermal fluctuation noise arises due to the high-frequency
thermal decoupling of the normal and superconducting phase regions inside the
TES film. Our results are also consistent with the prediction that in thin
bilayer proximitized superconductors, the jump in heat capacity at the critical
temperature is smaller than the universal BCS theory result.
