Usually caused by a weak battery or cable corrosion. Put your mower battery on a charger and when the charger indicates the battery is recharged, try to start. If the solenoid still chatters, try jumping from your car battery. If it cranks well with the car battery, then you should probably replace the mower batttery. If it still chatters, check the pos. wires twixt battery, to solenoid and solenoid to starter (both red) , as well as Neg. ground wire (black)
Gary Zerbst, June 2009