It is quite normal for heat pumps to make noises. This is especially true in the cold winter months. Most heat pumps have valves that carry out the task of reversing the flow of refrigerant when you switch between the cooling and heating modes. When the pump goes into defrost during winters, the valve is made to shift position. This is accompanied by a whooshing sound that persists for a few seconds. The compressor tends to get much louder after this. Another time when you may hear sounds is during shutdown when Freon pressure tends to equalize. Both of these are totally normal.
Another case where you may hear loud sounds coming from your heat pump is when the outdoor unit shuts off or starts running. Scroll compressors have a tendency to do this in a more noticeable manner. Similarly, some outdoor units tend to create a buzzing sound even when they are not running. This sound is created by the 24-volt reversing valve coil. Since this doesn’t waste any energy, it is nothing to be worried about. The sound of vibration is another common complaint and is quite difficult to get rid of. Rubber tubing can help in this case, but the vibration is not really doing any harm to your pump.
On the other hand, if you hear very loud sounds which resemble metal hitting metal, it may be time to worry. These sounds are often created when the fan blades hit against ice, wires or even tubing. If this is the case, you need to shut the unit off at once. This is a bad sign and can damage the fan and even the motor to an irreversible state. In fact, a misplaced tube can hit the fan blades and also cause a leak leading the Freon to flow out. Grinding or squealing sounds may point to a similar problem that needs to be fixed promptly. So if you hear any of these sounds, make sure you shut the pump off immediately and call for professional help.
Anything including low refrigerant charge, a faulty reverse valve, and defective motors or compressor valves can cause this problem. So make sure you get it checked by a Naperville heating professional if the problem persists.