gingertooth, 03'WRXDriva, rgprestige15 - We all have different sounds and i think different causes. I have been trying to diagnose mine for a long time, but maybe some of my experience can help ya'll.
For gingertooth, I think you should take off your heat shield for a while. Also check for clearance between your down pipe and the fire wall. Also look underneath the car for clearance issues with the cat back. A change up front can shift your cat back also.
For 03'WRXDriva, Yours sounds like waste gate flutter. Try to tighten the arm. Although i bet you already tried that. If you have the sound at full throttle too it might be my problem. I can hear a sound at low boost too. At similar times you hear yours. I think this sound is just do to my up pipe change. No Cat and the flex pipe i have could crate turbulent flow in the pipe i think.
For rgprestige15, If its only when you are idling i don't know. If it changes at higher rpm to a solid scream or whine it could be an up pipe leak. If this is the case get copper sealant and redo your Up pipe. If its a leak though it should get louder at higher boost.
I hope this helps. Sounds in this area are hard to diagnose.