Small Engine Troubleshooting Problems? Ask a Mechanic Now.
With spark plug wire removed, put it in gear and try to turn the belt that is squealing. Is it tight? Or does it slip on the pulley? It has to be tight, the adjuster has to keep belt tight. It can be out of adjustment, or the adjuster spring and adjuster may be stuck.
With spark plug wire removed, raise the drive, and loosen the belt, see if the drive is easy to turn, or is locked up.
Might try a new belt!
Sorry, but if belt slips when drive is use, and belt is tight, a new belt will have better grip. And you say gearbox is free!