iPhone volume dips for a period of time, then increases again

Very often; I.e. 3 or 4 times an hour, the output sound on my iPhone dips for a period of time, and then returns to normal levels.

This when I'm listening to Podcasts, Apple Music, Spotify, BBC Sound player etc. If I'm listening on earphones, bluetooth connected to the car, or just on the iPhone speaker.

I feel like it is linked to notifications. It certainly seems to be a dip in sound to accompany notifications, however I do not get any notification sound, nor does it seem to coincide with a notification coming through.

I have turned off all my notifications and no change.

It doesn't seem to matter what Focus mode I'm on.

And it is "random", often happening a lot in an hour, and sometimes not a lot. Occurs more often during the day, so I do suspect it is linked to notifications.

Sometimes the dip in sound lasts for 10 seconds, sometimes it lasts for 30 seconds.

I have tried turning of "Attention Awareness" features, no joy, same thing keeps happening.

I have a mate who has exactly the same issue, so it's not just me.

I went into the apple store and they did a hardware test. All ok, and said reset all settings or reinstall.

Before I go through all that hassle, any tips would be great. It's really really annoying.