Put on a new pulley bearing.
The roller bearing dates to at least 40 BC, and it was applied to pulley blocks sometime in the dim past of history. There is probably no way to know when someone first put a roller bearing into a pulley block.
In most cases you can purchase the bearing and pulley as an assembly from your local auto parts store.
Either someone has used a belt dressing that left a residue on the pulley or there is a bearing oil seal leaking in the vicinity of the pulley. A few more details of the pulley's environment would be helpful.
There is a small bearing in there and sometimes they just fail. The bearing can't really be changed so you'll have to buy a new or used idler pulley.
You can, for about 10% of the price of a new pulley, but you need a bearing press or hydraulic press to do it properly. A new metal pulley usually costs around $30, but the bearing can be obtained for $3 to $4.
Take off belt, remove bolt in center of pulley, replace with new pulley (comes with bearing). Reverse to install Take off belt, remove bolt in center of pulley, replace with new pulley (comes with bearing).
it is time to change bearing on that pulley
That pulley has a bearing on the inside of it that has gone bad. Replace the pulley with a new 1
If you're describing the idler pulley or the tensioner pulley, it's probably a bad bearing. Pulley bearing failure is common and relatively easy to repair. Remove tension from the serpentine belt then turn the pulley back and forth with your fingers. It should make no noticable noise and should turn smoothly. If you can hear any grinding noise or if the pulley feels like the bearing is grinding, replace it.
check you motor mounts and trans alignment your pulley shouldn't wobble with a new assembly
Bearing needs to be replaced