Thanks to Dave and Torin,

I am now aware of a thing that I can't actually hear! I developed tinnitus about 20 years ago, and, though it doesn't bother me very much, it does mean I can't hear sounds in certain ranges.

I think my microphone (a Samson C01U) is picking up the mains hum, maybe from my (now old) desktop or the fridge-freezer nearby. I recently moved the mic stand to the other end of the (shelf over the) desk, but although it seems to have fixed other distortions, it didn't do enough. I plan to remodel everything as soon as I can (probably over Christmas).

When I record my own shows I usually use my Zoom H3n recorder on a boom, and it doesn't seem to get much interference. I do sometimes use it for the Community News recordings, but I dropped it recently and broke the tripod mounting thread in the base, so it's not easy to use. I've 3D printed a possible fix, but haven't tested it yet.

I tend to run all my recordings through the noise removal filter in Audacity, but this particular show came from Ken's recording of the Mumble stream, so it would probably have been very difficult to clean up.

Anyway, thanks for the feedback (as it were).