I'm having this but mine isn't really fleeting. It was constant for the first day, then went to on off, then off for the day and on for the evening. Today it's mostly off, although it was on in the morning. It is scary. The first doc I saw said it is the carotid artery I'm hearing because it's on the right side. Also, if someone else can hear it that means it is external to the ear and can be serious. You should get it checked out just to be safe.