Howling sound when pressing accelerator
 

Have a 2001 Cherokee Limited that produces a howling sound when pressing the accelerator. Sounds like it is coming from the transmission but no sure. Only happens when the vehicle is moving, when accelerating sound gets loud, let off the gas, the sound goes away. When transmission is in park, there is no howling when pressing the accelerator. There is no vibration or shaking when driving, just the howling sound. Checked fluid levels in both transmission and transfer case and both are full. Any help would be appreciated.