I'm gonna go with mice.


Check all the trim and weather stripping around that area. Kinda sounds like something flapping around.
After a good rain or even a car wash, check up under the dash and carpet for any moisture. I seem to remember there was an issue with a seal leaking and getting all the electrical junk in there wet. That might have been on a D1, but these things aren't getting any younger so its not too far out there to look at something that's letting some wind in or has come loose.
Have someone ride with you and see if they can gently push and pull on the dash, glove box and panels or even push on the windshield. Or have them drive and you do it.

 

Thanks guys - went on a 4 hour road trip this weekend and didn't hear it at all so not quite sure. It definitely comes and goes. I'm going to do everything suggested here and will report back - thanks for the help!