Garageband noise gate issue - can't remove background noise from a recording

justynasowa wrote:

when I use the noise gate plug-in on the relevant track in Garageband I don't get any incremental reductions in the background noise as I slowly move the slider to the right. Although it does cut all the sound off when I move the slider farther over.

this sounds like you may misunderstand what a noise gate is and how it works.

in an ELI5 analogy: think of the noise gate as a tiny man with his finger hovering over a mute button. any time the signal drops below a certain level (the noise gate's 'threshold' setting) he pushes the mute button. as soon as the signal rises above the threshold he releases the mute button.

a noise gate can be very useful in cutting out breath sounds between spoken/sung passages, but it's not truly a noise removal tool. if there is a quiet hum, a noise gate could silence it between words, but once the level goes above the threshold (when someone is speaking) the noise gate unmutes and lets all sound go through.

if the hum is noticeable during spoken passages, or is too loud to use the noise gate anyway, what you might try is figuring out at what frequency the hum occurs (a 60Hz hum is a typical issue) and use an EQ to try to reduce it.

outside of that, Audacity is a free audio editing program that does offer 'noise reduction/removal' tools, that may be worth exploring.