Good evening!

I have recently bought myself a lovely little folding bike, its awesome, so far . However, i'm hoping a few of you more experienced people could give me a few pointers/help with a small issue i currently have with it, im hoping its just some initial tuning i need to do.

Firstly, the bike. i'm told it's a rebranded citizen bike (born2ride from germany), not sure if this is accurate, but it looks almost exactly the same as what they have on their website.
Here it is after a quick test when i adjusted brakes/seat



Now the main issue i currently have, is that quite often, when switching up into slower gears, the derailleur (?) makes a constant clicking sound until i change up and down again. changing into the faster gears seems fine however. Does anyone have any idea why? or how i could fine tune the gears to my liking?, i read that gearing on these bikes are different from other full sized bikes (derailleur is reversed?). so i'd rather get some advice from people who may have done this before.

Any help or advice you guys could throw my way would be really awesome
Thank you for reading!