Possible Drivetrain Problem?

[Kynetx]

I'm having some trouble diagnosing this one. Here are the details:

The noise is a rhythmic hammering that seems to be coming from the trunk area. The noise is noticeably louder under acceleration or deceleration.
The noise is only present when the car is full of luggage AND passengers.
It will only show up if the car has been driven for 30 minutes or more.
The noise is not accompanied by a loss in power or a strong vibration.
The U-joints and flex joint are in decent shape. There appears to be no play in the drive shaft.
Any ideas?