The creaking was driving me nuts! So I decided to analyze the problem. And in the process I found a simple fix, just adding a small piece of electrical tape...
I took some photos with my iPhone, somewhat documenting the process, see this gallery:
http://www.head-fi.org/g/a/1042689/nad-viso-hp50-creak-removal/
My process in words:
1. I removed the earpad - four metallic screws appeared.
2. I unscrewed the four metallic screws and gently removed the plastic part with the driver attached to it. There was a piece of loose black foam behind the driver - I put it aside.
3. The secondary black cover piece could be removed (not screwed in, but may not come off without some fiddling).
4. Four black screws appeared - unscrewed them.
5. Now I could see the hinge mechanism, which seemed to be the source of the creaking noise.
6. With some trial and error I determined that the creaking noise came from the encircled plastic parts rubbing against the last removed plastic piece (the one attached with four black screws).
7. I added a small piece of electrical tape to the problem area and the creaking noise was gone!
8. I put the parts back together again in the reverse order. When you get to the point of putting the loose black foam back in, note that the cord goes on the backside of the foam (towards the outside of the ear cup) and that the side with a circular indentation faces the driver.
Note the little self-adhesive foam pad in one of the photos. I bet that is there to prevent the hinge from scraping against the black piece of plastic! But the two encircled pieces of red plastic apparently also needed some kind of "padding".
I hope this is of help to others experiencing the same problem...