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

I am 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 is some smooth sound. But I have observed in HOnda bikes, there is no such thing… Engine sound is identical in all the gears… Please explain me

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