then it would just be thumbing once at the end of operation. The float and the way it makes contact with the switches contacts sound like they are screwy. Set too sensitive or dirty or something. Check the contacts and try to adjust the float if it is an adjustable kind and check for depress.
It sounds to me as though the motor is kicking on and off. If it is the water pounding back against the check valve you might have an air chamber or properly placed air vent installed to soften the blow.