Pully barring

Most likely your idler pulley and it is an easy replacement on most engines.

 

Take a spray bottle of water, squirt water on each pulley hub in turn, when the noise stops briefly you have found your culprit.

Usually idler, as ridgeline said.

Tensioner is also a likely suspect.

 

A 3' rubber hose. One end to your ear the other by each pulley. This helps isolate the source.
Same as above.

 

I used wd 40 with the straw. If it's a dd15. Spray from the back of the wheel. There is a real small gap to get the oil in. An idler pulley is around $40. The one with two pulleys and springs for tension that's around $280. Change out belts too. Becareful tightening it's an aluminum frame that's holds all of this.

 

The reason I use a hose is IF accidentally the hose is pushed against the rotating pulley. It bends. Also it's easier for me to see the end and where the sound is coming from.

Some noise you can start hearing midair with a hose.
Staff or rod works great pushed against the hard block or valve cover, trans case.

 

Just replace the idler pulley as a unit, it is also called a belt tensioner. These things have a life about 350k miles and really should be replaced with the second belt, which has a life about 150k miles