Whisle noise

The whistle noise is a lower sound whistle I thought it was coming from the engine compartment but after having other people riding with me with the windows down, it appears that it might be coming from underneith the cab. It might be comin g from the cataletic converter area. It can not be heard if reved up in park or in gear with the brake on. The vehicle has to be moving and seem to show up between 30 and 80 k. The vehicle has to have been running for a while, warmed up. The mechanics have checked all the intake system for leaks to the best of their ability. I started to notice the noise just in the last 4 -5 months. No repair has been done on exhaust, No new tires. Again, it is very noticeable while under load, pulling a trailer.

My 07 sierra sounded like a whistling tea kettle.I even bought my own smoker on ebay to check for vacuum leaks. Checked for leaks and there were none. After many hours of looking I decided to take the drive belts off and start the truck. My whistle was my ac belt tensioner pulley. I would have bet that not to be the problembecause I have never heard a bad pulley bearing sound like that. There was no play in the bearing. Hope my findings will help all you gm guys