Have you got a video of the sound ??After a while the engine starts to emit a creaking sound when climbing hils. Like there is some sort of cog or perhaps the belt i straining. The sound goes away when you pedal on a flat trail, but as soon as you put a bit of preasuer it comes back.
Just out of curiosity, what mode are you in eco/trail/turbo when this happens and what cadence does it happen at ?