Brought the car to the dealers, they seem to be puzzled by the strange noise as well, checked the exhaust pipe for leaks, none were found. However, the mechanic I spoke to did not take a look at the connection from the manifold to the exhaust. I was told that if there were any leaks at the exhaust, even if the car is not accelerating, noises could be heard. Actually, the "noise" comes on as a form of intermittent "puttering" sound. Really puzzled by what could be the possible problem. Usually, on accelerating, the usual sound heard is the gradual "vroom" from soft to loud. However, for my car, just before the "vroom" becomes loud, the "puttering" sound comes on and goes off. Is it necessary for the entire exhaust to be removed in order to have a thorough check to acertain whether the exhaust is leaking?
After much test-driving, I was told by the mechanic that nothing was wrong with my car. Seems like no one could give me a reasonable explaination about this "strange sound". Wonder if anyone here has a similar situation to mine.