Mr Foldy is a Dahon MU P8. He isn't a year old yet and has done around 2000 miles. I have never looked at the frame hinge, until it developed a ticking, clicking noise. It took a while to isolate it, but its definitely coming from the hinge area, even in free wheel. I posted below about tightening it, but without risking going too tight, it hasn't helped much.
The internet has some horror stories, but not much in the way of help.

I had a good look at the hinge and the clamp, there is nothing obvious in terms of wear. In fact it looks good as new. If I put the frame together but don't clamp, there is up and down movement. Clamping does make the frame feel completely solid in all directions and it always has done.

I'm going to thoroughly clean the clamp when its open, I usually clean the bike fully assembled. And I might have to think about my LBS.

Do I have a problem? Or just an irritation? I love my folder, but I could live without this worry!