Hmmm I have my own strong opinion about Dahon... I am convinced they try to cut corners. (EG it took a formally lodged complaint with the relevant safety authority before Dahon issued a recall on that dodgy handlepost, long after it became known as an issue.)

Regarding production quality, the do turn out huge quantities of bikes and the occasional quality issue will slip through quality control. They have a policy that a bike shop must give the bike a good once-over before delivery to catch any issues. But of course even the best bike shop may also miss items.

However, by and large I also think Dahon bikes are of much better quality that the Citizen-class bikes of this world. There are also some very polished products in their line-up. But the frame hinge gets a lot of heavy stress and my personal opinion is that aluminium hinges do not stand daily commuting + folding very well. Commuting is the heaviest duty that a bike can do, especially in bad weather. Aluminium being soft will deform and erode faster.

A creaking frame hinge is usually the result of the hinge faces moving against each other with stress. Only a very tiny amount will produce a very loud noise. A way to cure this is to place a layer of duct tape on one hinge face so that there is something between the metal faces. This will also stop erosion of the metal.