Why is my dryer squeeling when turned on?

Almost always it’s the belt slipping on the drum. Unplug it from power. Carefully pry the leading edge of the top off the lower. It’s hinged at the back, so swing it up and prop it to check things. The belt is under a lot of tension, check this and the condition of the belt. The drum should spin smoothly with no wobble. The support rollers age and will make the drum harder to turn and may wobble if really bad.

All of this is if the squeal is when empty. All dryers squeal under heavy load - full of denim. Over filled also makes it much less efficient.