[BR3W CITY]

Can you watch STFT? You can sometimes identify an exhaust leak based on fuel trims.

Honestly tho, it sounds like you have a certain exhaust pressure that leaks in a place or two. When its idling, its not enough back pressure/exhaust pressure to leak..and when you rev past that point, your creating enough outgoing pressure that its either forcing past the leak, or leaking constantly to the point where you don't notice it apart from your exhaust tone.