Yamaha DTXpress/DTXplorer/DTXtreme

Thanks for the advice, guys. I had tried the reset - without luck, and 
had run out of ideas at 2am last night. Back this afternoon, I noticed 
that the audio output was getting slowly noisier, on top of the click, 
and this is where the ground loop idea came in. The only thing I 
hadn't changed were the (brand new) audio cables, but it seems one of 
those had gone slowly bad (looks like a dud solder joint to one of the 
shields). Replaced audio cables - problem goes away. If the cable had 
just gone bad straight away (it worked fine on a test guitar), the 
midi complication wouldn't have come up; Murphy strikes again.
cheers
tomr