Find and remove hum or rumble

Find the frequency

Select a passage where the hum is exposed, such as a pause, and choose Analyze → Plot Spectrum…. A narrow spike at 50 or 60 Hz points to mains hum; spikes at multiples such as 100, 120, 150, or 180 Hz are its harmonics. A broad rise below about 80 Hz with no distinct peak is rumble from traffic, handling, or wind rather than hum.

Use the peak buttons or type a frequency to read the exact value, and close the dialog when you know what you are dealing with.

Notch the hum

1. Select the audio to treat, or clear the selection to treat everything, and choose Process → Notch Filter….
2. Press 50 Hz or 60 Hz, or type the frequency you measured. Start with a Q between 2 and 10; higher values remove a narrower band.
3. Apply, listen to a passage with voice, and plot the spectrum again.
4. If a harmonic remains, reopen the Notch Filter with that frequency and apply again.

Each notch removes everything at that frequency, including wanted sound, so treat only the peaks you can see and hear.

Clear rumble and check the result

For low rumble, choose Process → High-Pass Filter… with a cutoff around 80 Hz for speech and a 12 dB-per-octave roll-off. Raise the cutoff or the roll-off only if rumble remains, and undo if voices sound thin. Compare with headphones, then save the project and export.

If noise is broadband hiss rather than a steady tone, Noise Reduction with a captured profile is the better tool.