  We report on measurements of dissipation and frequency noise at millikelvin
temperatures of nanomechanical devices covered with aluminum. A clear excess
damping is observed after switching the metallic layer from superconducting to
the normal state with a magnetic field. Beyond the standard model of internal
tunneling systems coupled to the phonon bath, here we consider the relaxation
to the conduction electrons together with the nature of the mechanical
dispersion laws for stressed/unstressed devices. With these key ingredients, a
model describing the relaxation of two-level systems inside the structure due
to interactions with electrons and phonons with well separated timescales
captures the data. In addition, we measure an excess 1/f-type frequency noise
in the normal state, which further emphasizes the impact of conduction
electrons.
