Anyway, I keep heard the 'clark clark'- sound, any idea on which part faulty?

The XDS K3 looks good. They do build solid frames but because of human's chase for branding, they are not popular compared to the Terns or Dahons. The XDS K3 is more than enough for the job that you described. Like I said, if branding is not an issue, or the size of the fold isn't, it is more than enough.

BUT !

If you, one day will to join a biking group, riding here and there weekly, that's where the problem comes in. This is a big IF lah! Peer pressure will make you want to change bikes, not the performance of the bike or the quality but usually it is the peer pressure! 

After riding for maybe total of 40-50km, now feel like the Cassette  9 speed not enough

Any recommendation for upgrade on Verge D9, think is using Shimano Acera

Anyway, I keep heard the 'clark clark'- sound, any idea on which part faulty?

happy tuesday everyone!

I think you need new 56t crank...

56t, hmm maybe is good upgrade. is this going to be from China / CNC version? as I don't see Shimano having his much "teeth"

I am pulling trigger for one soon..16" too cute to resist!

and what is ur choice bro?fnhon?

is that possible the crank loosen?
Last week i heard the similar sound when i turn the paddle, found out that my crank bolt loosen. (square tapered, not hollow type)
dont know how the hell that can happen. You may check this. 

56t, hmm maybe is good upgrade. is this going to be from China / CNC version? as I don't see Shimano having his much "teeth"

sudah kena fnhon racun........

Just buy the 55T or 56T chainring from BBS and then change it. See if the existing chain can accommodate. If cannot , tukar lagi.

Square tapered cranks and BB, they are actually very torque critical when tighten. Last time, I did not have a torque wrench and instinct made me lock tighter and when once over doing, gone. When you enlarge the taper square hole on the cranks, it will loosen a bit after hard pedalling and that's when the clak-clak clak sound comes. I need to carry a number 10 allen key last time as a precaution.

After riding for maybe total of 40-50km, now feel like the Cassette  9 speed not enough

Any recommendation for upgrade on Verge D9, think is using Shimano Acera

Not enough to climb hill or not enough to go down hill or not enough fast for flat?

Climb hill and fast straight road.

Double or triple chainring. Will get you sorted out. What is your current chainring and Cassatte size?
How many gear step up and down you think you lack.