2004 1.8t, quattro, avant, 60k miles...

i have this annoying "whistle" around 2000 rpm, only when on the brakes or just off the gas. it's not the dentist drill sound that some say when a turbo is failing, but just a "chirp" when decelerating. under the hood it sounds like it's coming directly from the turbo, but it only happens after 15 minutes or more of driving. no sound at all when started cold. what is it? is my turbo really failing? my guess is a gasket or hose. any help is greatly appreciated!