We present a cryogenic microwave noise source with a characteristic impedance of 50 ω, which can be installed in a coaxial line of a cryostat. The bath temperature of the noise source is continuously variable between 0.1 K and 5 K without causing significant back-action heating on the sample space. As a proof-of-concept experiment, we perform Y-factor measurements of an amplifier cascade that includes a traveling wave parametric amplifier and a commercial high electron mobility transistor amplifier. We observe system noise temperatures as low as 680-200+20 mK at 5.7 GHz corresponding to 1.5-0.7+0.1 excess photons. The system we present has immediate applications in the validation of solid-state qubit readout lines.