Siri on the iphone waking up and listening? (not sure if android has something similar) That will lower the volume on the radio, and gives no on screen prompts if you're bluetooth connected.. Siri wakes up more now with ios 13 than it did before.Mine sounds more like a sonar chime. Also this lowers the volume of the radio when it pings then the volume returns. ?
anything connected to audio, or alerts from the NAV unit (seems like some alerts like low fuel when navigating do that, but you get a big screen you can see).