If the outside casing is slightly blackened, by rust or wear, you should avoid using it. However, many people have suffered severe consequences because of lack of recognition of inner damage, particulary in commercial models.
Call the parts store. They will be able to tell you if the pulley is available separately or not.
Assuming your serpentine belt is in good condition... the bearing of pulley could be making noise (or bearing could fail allowing you to jiggle pulley with your hand), and/or tensioner spring is worn out.
That pulley has a bearing on the inside of it that has gone bad. Replace the pulley with a new 1
I'm assuming you mean a pulley, which is what the belts run on. If so, depending on which pulley the bearings could be bad or if it is the crankshaft pulley (big pulley near the bottom) the rubber could be getting bad.
When it goes bad
In most cases you can purchase the bearing and pulley as an assembly from your local auto parts store.
Serious vibrations.
cause the tensen pulley could be going bad an my need to be replaced or even your alternater pulley can be getting ready to go bad
The idler pulley has nothing to do with the brakes. If the idler pulley is bad it could throw or break a belt, and then your engine will stop.
All pullies have bearing in them.Grab the pulley and try to move it up and down.Excess play means bad pulley.Watch pulley to see if it turns free and true.
Remove the belt(s) and manipulate the pulley by hand; its action should be butter-smooth.
If you mean the pulley and the shaftit is mounted on, Yes! A loose belt on the pulley doesn't necessarily mean anything but a need to adjust the belt tension.