As mentioned in the comments, the Cosmic Microwave Background creates noise. More specifically, cosmic background creates blackbody radiation, which adds kTB noise to any receiver looking at it. It's called kTB noise, or thermal noise, because its intensity is the product of the Boltzmann constant k, the temperature of the blackbody T (around 4K for the cosmic background) and the Bandwidth B of the receiver.
A good explanation on kTB noise and receiver sensitivity:
This (https://descanso.jpl.nasa.gov/DPSummary/Descanso4--Voyager_ed.pdf) NASA pdf on Voyager telecommunications gives the following sources of noise for the downlink (Voyager to Earth) (from page 26):
Noise power of each source: Design value/Favorable Tolerance/Adverse Tolerance
Total Noise Spectral Density, dBm/Hz: –185.35/–0.97/0.80
Total System Noise Temperature, K: 21.12/–4.24/4.24
Receiver Temperature, K: 13.20/–3.00/3.00
Ground Contribution, K: 2.88/–3.00/3.00
Galactic Contribution, K: 2.68/0.00/0.00
Atmospheric Contribution, K: 2.36/0.00/0.00
Hot Body Noise, K: 0.00/0.00/0.00
Elev Angle = 58.01 deg
So the major source of noise is the receiver, followed by the blackbody radiation from the ground, from the Milky way and from the atmosphere. The CMB seems to be negligible compared to that.