Hero Honda Passion Plus
Whistling sound from the engine checked by the service person many times but still the same

Respected sir,

I bought my Hero Honda Passion Plus motorcycle on 21 august engine no 07D08M44297 and chassis no 07D09C33864 from Byke Point an ARD of Mark Motors, Near Railway Gate, Perinthalmanna, Kerala. 5 Free services where completed after the third service i came to notice a whistling sound from the engine so i took my bike to the authorized service center they checked it and said its not a big problem the sound is from the fuel re-burning tube but am not getting the sound always. Now 5 free services where completed but still am getting the same sound from the engine so please give the necessary solution for this.