Hero honda/Spelander Plus
Problem not resolve

I have purchased my bike dated 29-01 from Khanna Auto Mobile Tilak Nagar.

I Have made service at Khanna auto Mobile (Maya Puri).

after 1st Service bike making a sound "Fattttt" from Silencer (Back firing).

at the time of 2nd service i told to the service eng. about the Prob. but prob. was same.

At 3rd service i told to the manager of service canter. he told me that the issue will be resolve.

But the Problem is same..