How do you know that it is stock ? have you owned the car from new?
If SMF fitted. then that is your cause.
Otherwise if definitely stock, then could be DMF, exhaust baffle, exhaust rear box heat shield, cat heat shields, however could be many, many other things as not enough information provided.
If you are up in Perth at any time, then just swing by and I will soon tell you what it is and also the fix.
Alex.