[Sans Peur]

There are several thing behind the dash that may make noise. High water alarm, oil pressure, water temp, even your GPS . If something is getting a little weak it may allow a small amount of current to go to the piezo alarm. This will make it a muted sound unlike full voltage going to it. Since you switched off everything one by one and only a full disconnect of the batteries seemed to work I would look at systems that are usually wired directly like the bilge high water alarm.
Also look to see if you have a co or smoke alarm wired in. I had my co detectors getting older and they would sometimes go off. A couple of times one of them would just kind of chirp a few times.