Any idea how to get rid of aircraft background hum without altering voice quality? It's for footage filmed aboard an aircraft.
If it's relatively constant, then use a sample of it with no speech as a profile for Audition's Noise Reducer. But you'll have to be careful about how much you remove, because this is very much in the same frequency band as speech.
If you are not familiar with Audion's noise reducer, you may find an expansion of Steve's brief account useful - he knows Audition inside out!
If you view in spectral mode, the aircraft noise will show as a bright band across the screen at a frequency in the hundreds of Hz - right in among the speech frequencies as Steve says. Find a short section of the noise where there is no speech or other sound that you want, and drag a marquee around just that section (you may need to expand the spectrum vertically to do this effectively). R click on your selection and select Capture Noise Print. Now open the Noise Reducer, click on Select Entire File and run the preview. When you are happy with the results, click apply
You will achieve the best results if you run two or three passes at lower percentages of reduction than one at 100%. Increasing FFT size can also improve your results.