Hit "Play" and listen to the track.
Hit "Stop" when you hear the offending sound.
Click one bar left of the time marker, on the time bar at the top of the recording interface, and drag the cursor to the right. This creates a loop, which means you don't need to keep stopping and starting the track.
Hit "Play" and let the loop roll.
Click "S" on each channel and listen to one loop. This solos the track, muting all other audio. This is the best method to identify a one-off sound in the mix. By muting all the other tracks, you can identify the audio channel on which the sound occurs. Repeat the process with all tracks until you identify the audio containing the unwanted sound.
Double-click on the audio channel in question. This brings up the audio as a sound wave graphic. The peaks represent volume.
Click "View" and select "Zoom." Identify the peak that corresponds to the unwanted sound. Typically, an unwanted sound occurs in the background and the peak is smaller.
Click "Tools" and select the trim tool. The precise command for selecting this tool varies according to which program you use, but it is typically denoted by a scissors icon.
Click the cursor on each side of the sound wave. This isolates that part of the audio from the other audio in the channel.
Click on the selected audio. Click "Delete" to remove it permanently.
Play the track.
Click "Tools" and select "Equalizer." This opens the equalizer interface in a new window. An equalizer contains a series of slider dials and a grid interface, and it lets you boost and cut frequencies within the sound. If you have a persistent, low-level sound that requires removal, rather than a one-off glitch, equalization is the best way to tackle it.
Click on the equalizer curve and drag it around the grid interface. The vertical axis represents volume, and the horizontal axis represents frequency. By moving the curve, you highlight a variety of different frequencies. When you hit the frequency of the unwanted sound, it will become loud and obvious. For example, if a guitar amp is humming, the hum will dominate the mix when you maximize the relevant frequency.
Release the mouse when you hit the frequency.
Click "Gain" and drag the level to zero. This effectively mutes the frequency on which the unwanted sound existed. Repeat this process to remove all relevant frequencies.