Check the gearbox for oil leaks. Check the oil level in the gearbox by removing the fill plug. If the oil is contaminated, drain the oil and refill with new.
Remove the fill plug and check the oil level with your finger.
The oil should be at the level of the lower edge of the filler hole (fill plug holes). If the oil level is lower than specified, add oil until it starts to flow out of the hole, then reinstall the filler plug.
Change the gearbox oil if it is dirty or does not have the correct viscosity.