I would guess you are hearing the blower wheel.
Blower Wheel
They are plastic and get stripped out in the center were they connect to the motor.
If you remove the front of the dryer you will see the blower wheel.
Try to give the wheel a spin by hand and see if it's lose on the motor shaft.
If it is then replace it.
If the blower wheel is fine I would check the rollers the drum ride on.