  In this letter, we address the quasiparticle dynamics in thin aluminium,
which is critical to improve the sensitivity of superconducting detectors and
the coherence time of qubits. We have measured temperature dependent
quasiparticle fluctuations in a small Al volume, embedded in a NbTiN
superconducting microwave resonator. When the quasiparticle lifetime saturates
at low temperatures, we observe a noise level reduction of a factor $\sim$100,
from which we deduce that the number of quasiparticles does not saturate, but
continues to reduce exponentially with temperature. Comparing resonators on
substrate and membrane, we show that the saturation lifetime does not depend on
phonon trapping. Quasiparticle trapping is consistent with this behavior, when
on-trap recombination limits the observed lifetime. Counter-intuitively, this
effect improves the sensitivity of single-photon detectors.
