I have an 08 GMC Sierra 2500 HD with about 85,000 miles on it. Noticed a weird whistling noise the other day while pulling out of a parking lot. I only hear the noise around 1000 rpms. I thought it might be the water pump, idler pully, or tensioner so I took the serpentine belt off to see if the noise would go away and confirm my suspicion. I started the truck and got the rpms up around 1000 and the noise is still there. Not sure what else it could be. Any help or advice would be greatly appreciated. Thanks