Kelvin,
Have anyone experience this before?
Recently when I reverse the car, I have accidentally hit the exhaust to the kerb (quite hard). I went to check, nothing seems wrong. When I drive, I notice the sound seems like coming from the exhaust. So I try to off the air-con and the sound seems gone. On it back and the sound back back. Not sure is it due to the exhaust pipe hit to the kerb!
Maybe drop by one these day to have a look?