First thing that comes to mind is a LOOSE fan belt. At the belts longest stretch, see that it ONLY has a 1/4"-1/2" play.
A loose belt will make a high pitched squeal that goes away when the belt isn't slipping; usually at higher speeds.
Also, check the pulley bearings.
The way to check is to get a LONG tool and CAREFULLY
place it on the pulley bolt when the engine is running. You can use a wooden dowel as well. BE VERY CAREFUL & ALERT
. . . it's a running engine.
Put your thumb on the other end and place your ear into your thumb. You will definitely hear a grind if its bad. They TOO squeal when they are bad and get HOT.
Do not replace your metal pulleys with a cheap plastic pulley. Remove yours and simply replace the inner bearing. it is an easy fix. Once you have the bearing replaced, if it is that, you will be surprised at the difference in noise.
It could also be your clutch/flywheel viewing shield if the noise is down below. The shield bolt might simply need to be tightened.