Mastercool evaporative cooler. Motor works fine however makes a high pitched noise/vibration in high setting?

Question:The motor operates fine. When placed in the higher speed there is vibration and unusual noise coming from the unit. Checked the fan belt tightness and it seemed okay. All of the attachment fittings are tight.
The noise coming from the unit was somewhat sudden and I am not sure what else to check or where the problem might be coming from.

Answers:
Have the outer bearing checked on the fan, sounds like when it heats up it is trying to seize