So, the HomePod is still making that dreadful noise whether it is playing music at the time or simply plugged in but not doing anything else. I read about re-setting it and tried that but it didn't work. It wouldn't reset on the Pod itself and when I went into Home App on my PC, it said it wasn't responding and to check its internet connection and/or wifi. I did and it showed it was connected to the wifi but it still wouldn't let me reset it either via the HomePod or via the PC.
I tried removing it from the Home App, it went away onscreen but then when I asked it to play it did, 10 minutes in it made the abrupt noise and stopped playing the music. Siri didn't respond for about 10 minutes and then it did. It played Classic FM for another 10 minutes, then the noise happened etc etc. It's now switched off at the wall and so useless. I reckon it's been behaving like this since about 4 weeks after the Apple warranty expired in May 2021.