All cars resonate with only one window open (air collides with air). Unless your other cars had poor air sealing (and the air coming in the window was able to escape from a different hole/window.
Since getting our 75D in March, which doesn't have a wind problem btw and we have the sunroof, I've read posts on here with people saying wind noise in their car came from seal issues, roof glass not being level across panels, and one guy had some issue with his door handle and the resulting wind getting inside behind it.