I'm using hero splendor bike pro for 3 years, my bike is ok except for it's peculiar sound?

I'm using hero splendor bike pro for 3 years, my bike is ok except for it's peculiar sound? - 1

Added (1). i have been telling this at the time of service, but they say no problem with the bike. Only while in 4th gear there's some smooth sound. But I have observed in HOnda bikes, there's no such thing… Engine sound is identical in all the gears… Please explain me

Yes, its okay.

Don't take it to the service guys, they know nothing
find a good bike mechanic in your town/area and
tell him the problem
he will most probably open the whole gearbox and
engine to solve the problem