After noticing unwanted sounds and problems I have planed the following maintenance and upgrades.
Friction and crackles in direction: new headset in view!
After having unmounted, inspected and cleaned, re-mounted the headset parts (see my posts in French and pictures here to understand why you need to inspect this part), I deducted headset bearings was completly dead. Then I have decided, first to order for new Dahon at Trinton Cycles, UK. Actually my assessment of this online seller is rather negative, I will write a post about my experience about Triton Cycles. For several reasons I have ordered a LitePro headset having read the super-documented article How to Install LitePro Headset for Dahon Bikes from handsonbike.blogspot.com. This website is a gold-mine for Dahon upgrades !
Annoying squeak noises coming from the rear axle
Complete cleaning of jockey wheels and chain.
My freehub is noisy!
Unmounting, inspection and grease-operation planed!
Upgrade: new pedals
Thinking for a new seatpost
In my opinion the original Jestream EX 33.9 Kore seatpost is too long with its 580mm and add extra weight. I'm thinking to replace it by a carbon seatpost or a shortest 31.6 aluminium Kore seatpost I have in my stock.