Does anyone know how to change the Android Auto Notification Volume? Mine is so loud and I can't figure out how to change it.

 

When a notification comes up,(such as directions from Google Maps), use the volume knob or steering wheel volume button to reduce the volume.
You will see the normal volume slider come up on the displays, but you are only changing the notification volume. Once the notification is complete the audio will return to your previous "normal audio" volume.

Until you get used to it, try to not many any volume changes for a second or two after a notification comes on/in. That is how I have accidentally raised or lowered the notification volume in the past.
Hope that helps.