The phenomena of background noise generation in transonic wind tunnels are analyzed. In unsteady aerodynamic tests, background disturbances create problems because the instrumentation senses them in addition to the fluctuations generated by the aerodynamic flow over wind‐tunnel models. Calibrations of 25 facilities show that the amplitudes of the background fluctuations are excessive. In porous‐wall facilities, one noise source consists of porous‐wall edgetones that are generated by the passage of air over the holes. A flow model is developed that yields the frequencies of these oscillations. Test section boundary layers and hole configurations govern the occurrence of these fluctuations. In slotted‐wall facilities, a noise source is the slotted‐wall shearing caused by the air flowing along the openings. In both types of wind tunnels, other flow mechanisms also cause disturbances, and these phenomena are described. Noise‐reduction methods are outlined which will provide valid unsteady aerodynamic results.