My 02 trail blazer makes a loud grinding/ whining noise when I let off gas?? Not the spline gears,what elese could it be?

Have you checked the brakes? The pads or rotor may be worn down. Wheel bearings? What I do, is find a quiet street or lot and drive with all the windows down, all noise makers turned off and listen to the sound that is made reflecting off a building. It may amplify it, and help identify the side it's coming from, and if it is a wheel area noise. Also have someone walk/run beside the vehicle to help isolate the noise. Does the noise go away when applying power, or putting on the brakes? Is it coming from the engine area? A water pump bearing freezing up? Alternator bearing going out? Try to isolate where the noise is coming from and then try to do a visual exam for any damaged or worn parts.