I seem to always have to doozies. The phone will buzz, beep, buzz then the volume level popups on the left and is appears. I have checked my notification settings, checked to see if any pages open in Safari, closed all background apps, and updated to latest version. I also cleared my past notifications and turned off sounds. I even muted the volume in the pull down and used the side volume buttons.
Google has nothing of any help, have to come to the pros.
Thanks and have a great day!