SuperNova

I can only hear this when the windows is down and the sound is bouncing off something like passing cars, underpasses, etc. It's going to be spring and summer soon and I'm going to start hearing this again, it's so annoying. Does anyone else have this problem or have any ideas on what this would be? I've crawled under the car and I've found no obvious leaks or holes. How does an exhaust shop look for leaks anyway? Thank you in advance for any help possible.