Hi, How do I remove hum with notch filter and how to reduce noise using noise reduction and noise gate? Please help me!
Audacity Version: 3.7.3
OS: Windows 11
If the noise is bad, sometimes there’s nothing you can do, or “the cure can be worse than the disease.”
Noise reduction works best when you don’t REALLY need it. 
A noise gate kills (or reduces) the audio when the volume falls below the threshold that you set. It can work great for killing the noise between songs, etc. It can also work between words & sentences (with spoken word when there’s no music) but killing the sound between words/sentences can sound strange and be distracting unless the background is very low where it’s not that noticeable anyway.
If you have hum or a single frequency (or a few frequencies) look at the spectrum of the noise to identify the noise frequency to use for a notch filter.
There is a new AI Noise Reduction Plug-in which I have not tried.
Hum is be a series of horizontal lines on the spectrogram: here’s how to notch them out …
https://manual.audacityteam.org/man/spectral_selection.html#example