Yamaha DTXpress/DTXplorer/DTXtreme

Hi;
I have just started using my DTXpress as a sound module connected to 
my PC via an Emagic AMT8 interface. Whenever any midi message is sent 
to the brain, there is a click in the audio outputs. This click is 
there even when the unit is off, so I doubt its the settings. I have 
changed cables, midi ports, controllers, drivers for the AMT8. I have 
gone thru Logic Audio and not, but the click is still there. It only 
happens to the DTXpress, not any of the other devices attached to the 
AMT8.
Anybody else had this problem? any help, much appreciated!
cheers
tomr