1990 5.9l Cummins moaning sound
 

My Cummins just started making a moaning sound. Sounds like driving on a rumble strip without the vibration. It only happens at a certain rpm in any gear on any road surface. It stops if I let off or continue to go faster. I can hold the accelerator and keep the sound constant. It seems to be maybe turbo related but hopefully someone has had this problem. Thanks