Strange static/hum only happening sometimes within a file

Hi there! I’ve come to these forums a lot via Google when investigating various issues and they have been immensely helpful, but all my searching has not been able to help me with this one - I use a Zoom H4N with an ATR2100 to record my show (but started having this same issue with my ATR 2100 plugged into my Dell Inspiron Laptop prior to buying the Zoom). And I’ve found that there’s this strange underlying hum that Noise Reduction doesn’t take out because it’s under the words as well, and it doesn’t happen the entire time - I record a podcast that is anywhere from 25 - 45 minutes, and the hum usually starts at some point after 15 minutes in, and sometimes stops before the end of the show.

I apply noise reduction (using the ten or so seconds I leave silent at the beginning of the record), normalization, and a noise gate. I tried fiddling with the notch filter and couldn’t get it to do anything meaningful for this issue.

This happened on the version of Audacity I was using before, but now I am using 2.4.2 on Windows 10.

I’ve attached an audio clip where you can hear that it was not happening for the first 5 seconds (and prior 25 or so minutes), and then suddenly started happening and continued through the next 10 minutes or so of recording.

I am truly at a loss, so any suggestions would be much appreciated. I think what throws me off the most is that it doesn’t do it the whole time, so something I’m unaware of is changing at some point in the file/recording.

It sounds like that audio clip has already been processed, which makes it extremely difficult to diagnose what the original problem might have been.
If you still have the original, un-processed recording, we could take a look at that.

Ah good call, sorry about that! Attached is from the original file, with some of the non hum and then the hum right where it starts

The Notch Filter will remove most of it, but you need to run it three times.

This is the “track Spectrogram” view (see:
The panel on the left shows the settings. I also amplified the track (default settings) to make the waveform bigger and the spectrogram clearer.

Notice the three horizontal red lines - that’s most of the hum.
The frequencies are about 60 Hz, 24 Hz and 470 Hz (that last one is a bit surprising).
There are actually more than 3 lines visible, but those three are the strongest.

Running the Notch Filter at those three frequencies and “Q” set to 8 will remove most of the hum with little effect on the remaining audio.
Follow that with a little bit of gentle noise reduction to reduce the hum and hiss a bit more.

Oh wow, thank you so much for figuring this out - I was definitely not interpreting notch filter correctly, this is so helpful.