I have a rattle in my driveshaft.  It sounds like a bunch of pennies being shaken in a coffee can.  When the driveshaft is tapped with a ruber mallet, it resonates.  There is no vibration while driving.  

The driveshaft just had new u-joints installed and was dynamically balanced.  This completely cured a very bad vibration problem.

It acts as if the shaft has one tube inside the outer tube and the two are rattling against each other.  Is this possible?   Any other ideas?

Budd Premack
86 Syncro, 88 Wolfsburg
Land of Sky-Blue (Frozen) Waters
Minneapolis, MN