Could it be air getting in somewhere? Like maybe something that is not closed all the way?
I don't recall where it is, but there is a setting to warn you when you go over your designated speed limit. For me, that WAS 5 mph over the speed limit. But it was too darn annoying so I turned it off.
Check the Autopilot or Driving settings.
Just went to check - it is under Autopilot settings - speed limit warning. Can be display or chime.
Speed limiting warning
Switch from Chime to Display