Got my car back and the culprit is what you guys have already said. There is a dent and small crack on the exhaust pipe and that's what's causing the sound. They said that they have done their best in clearing out the dent and have also applied some sort of sealant. Despite their efforts, there is still some sound coming up although it is a lot less than earlier. They said that either entire system will have to be changed or the damaged portion welded at a workshop for the sound to be cleared out completely. They don't seem to have the required machinery for this work so I may have to go to a local mechanic. I don't think the sound is audible inside. Will try to take a pic if possible while checking wheel alignment. What surprised me is that the last day I had scrapped the underbelly only slightly while I have had a lot more harsher underbody hits earlier. So be cautious always guys. Another issue that I have in my car is with the speedo meter. Speedo meter is having a zero error of 8kmph. When the car is switched off all meters have their needles below the lowest Mark except the Speedo which has the needle at 0 kmph already. When the key is turned to MAR position all needles aligns with the lower Mark while the Speedo goes up above 0 kmph. Since they said they will have to open up the cluster I have kept it pending fearing it might bring up more issues.