I don't think it's the engine or suspension. My best guess is the drivetrain. Of course, my instincts usually point me in the direction of the simplest cause, first. Hence, my suspicions on the flex coupling. The rear one seems to be in good condition. I will inspect the front one this weekend when I change the oil. So, will a worn out coupling produce such a noise? I don't know if this will help, but the noise only comes out after I've driven the car for some time. This never happens on my morning commute.