poison alert!!!
the beauty of zephyr with 349 wheelset

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

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.

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.

Tern D9 comes with 52T 1x gives 28-93GI (gear inch), so 53T don't make much difference, a realistic 56T @11t will give approx 100GI would be nice, 60 is ideal but don't see them selling very often.
39T @36t will give about 21GI, can climb up a vertical wall liao
56/39 double chainring gives 21-100GI and more higher cadance workout
I assume you consider 30km/h as "fast". If you want >40kmh, get a kapchai.