WD-40 is a wonderful product with many good uses; however, your furnace blower motor is not one of them or any other motor. WD-40 has some sort of silicone base and dries out quickly in a motor leaving it to burn out the bushings or bearings. I use the Zoom Spout oiler for motors, it can be picked up at heating and airconditoning shops, sewing shops or big box stores.
Having a gas furnace leads me to believe you also have a gas water heater which could be heard when it kicks on and off. This short cycling is a big waste of energy and is probably caused by the thermostat. Try turning the temperature down 5-10*. You may also need to drain the water heater down some to get rid of scale in the bottom. If you hear your water heater rumbling when it is heating, it is the water boiling and bubbling up through the sediment. The only other thing I can think of that would affect the temp of the water and cause short cycling is a dip tube that has broken loose or disintergrated. The dip tube is on the cold water side and carries the coldest water to the bottom to be heated, while the hot water rises and is used first while the wh is heating more at the bottom.
You can find the dip tube by taking the fittings loose on the cold side, sticking a finger in the hole, bend the finger and pull. You should come up with a 3' long tube of plastic or copper. If there is none you can use a 3' piece of 3/4" copper with a flare on one end (that's what keeps it from falling down in the tank).