Sounds like an exhaust leak. Check the exhaust manifold for cracks and check around the gaskets (you may have to remove the heat shield to do this). A quick test is to hover your hand around where the flanges are to see if you can feel the exhaust gases escaping (just don't actually touch it as you'll burn yourself). If the leak is around the back of the flange then this test won't work, in which case you'll have to pull the manifold off and inspect the gaskets.
A good way to easily find a leak is first thing in the morning when the car is cold, as mist from the condensation usually escapes out of the leak which makes it a lot easier to see. Small leaks tend to close as the engine gets hot and the metal expands, so try and find it while the engine is still cold. Gently revving the engine via the throttle butterfly will help narrow it down too. Fortunately gaskets are nice and cheap, plus they're easy to install. Your car will be back to sounding how it should in no time :thumbsup:
A good way to easily find a leak is first thing in the morning when the car is cold, as mist from the condensation usually escapes out of the leak which makes it a lot easier to see. Small leaks tend to close as the engine gets hot and the metal expands, so try and find it while the engine is still cold. Gently revving the engine via the throttle butterfly will help narrow it down too. Fortunately gaskets are nice and cheap, plus they're easy to install. Your car will be back to sounding how it should in no time :thumbsup: