For the first time since June 2016, my truck's SYNC system dinged me constantly about new text messages. Apparently, I suddenly have the capability to receive text message notices and also, somehow, listen to them. It's annoying.

 

You can tweak it in the settings. I have mine off because nobody ever seems to text me unless I’m driving… and it is very annoying.

 

If your iPhone just did the ios 15 update, I'd bet some setting changed there.