For those of you with the TXS stealthback I'm not sure what you are hearing is the turbo whistle. I get the whistle between 1000-2800 rpms then it fades out and blends into the exhaust note. I've checked for exhaust leaks with a lit match and didn't find any. I also used plenty of muffler cement just in case. I wanted to find exactly where the whistle is coming from so I layed under my car while my wife blipped the throttle. I could hear the whistle in the rear section of the midpipe and somewhat in the front section of my Prodrive axelback. I do not believe what you are hearing is the turbo. I think it is just exhaust noise. If it was indeed the turbo then wouldn't the sound be coming from the turbo instead of the rear section of the exhaust? This whistle has been driving me crazy because most of my driving is done in rush hour stop and go traffic next to those concrete barrier walls which amplify the sound. Your feedback is welcome..............