So finally traced the issue. Listing it below for those who run into similar situation
The car came in a 40 foot container which the dealers get to save costs of transport. To fit maximum cars in limited space they lift cars up and put them on frames.
Due to this the undercarrige suffered minor damage and the silencer was moved from its location leading to an increase in the noise.
Got the plates yesterday evening and got it checked and fixed today. No more noise and all is well. Now drive even more better
Lesson: Import car seperately is possible, and if not then insist yours is the one on the ground.