I realize it's an old topic but luckily it came up in a search.
I am now experiencing this wonderful hissing/whistle noise. It only happens going from stop > start up to 1500rpm. As soon as I go over 1500 it stops and it is very faint. Some days a little louder than others but it's never constant.
I have been researching and I heard it could be the tensioner pulley or even PCV valve. I've had a listen and it doesn't sound like the PCV valve or pulley. It sounds more like it's coming from under the engine...
I just got back from the dealer with a very unhappy impression. Here in Canada it is extremely difficult to find a GM dealer that actually makes an attempt to fix your problem. Most of the time if you let them, you will walk out with everything else fixed but the problem and a empty wallet.
Anyway, can anyone suggest what to do? Ask them to replace the exhaust from the flex pipe? This sounds like a huge expensive job. If it's nothing too serious, I might just wait until it gets worse.
Thanks in advance.