... I just drove to SEMA and back a couple weeks ago and was experiencing the same EXACT thing.. but I think mine was rattling from both the driver and passenger side.
Driver and passenger side. Top of the windshield. Rattling noise. Cruise control around 75 MPH.
I just thought it was my Z falling apart because I drive it probably less than half a dozen times a year.
Do you think it has something to do with the windshield and how it's sealed? I thought it was because of chassis flex and the windshield being messed up because of that.