First off.. just start the car listen with the hood up. Move around the front of the car.. getting on the ground and stuff to see if you can roughly figure what are the sound is coming from.
You can check/replace the exhaust with a floor jack and jack stands. Just jack up the front of the car and support it with the stands. Do a visual inspection... look for a hole in the pipe, or see if the cat has been bashed in. If it is a leak, you should be able to see black carbon deposits around the leaky flange. Also check all the flanges to make sure all the studs and nuts there.
