  Thermal microwave states are omnipresent noise sources in superconducting
quantum circuits covering all relevant frequency regimes. We use them as a
probe to identify three second-order decoherence mechanisms of a
superconducting transmon. First, we quantify the efficiency of a resonator
filter in the dispersive Jaynes-Cummings regime and find evidence for parasitic
loss channels. Second, we probe second-order noise in the low-frequency regime
and demonstrate the expected $T^{3}$ temperature dependence of the qubit
dephasing rate. Finally, we show that qubit parameter fluctuations due to
two-level states are enhanced under the influence of thermal microwave states.
In particular, we experimentally confirm the $T^{2}$-dependence of the
fluctuation spectrum expected for noninteracting two-level states.
