You don’t need to answer the door. That knocking you hear is the result of a broken spring in your refrigerator's compressor. Previously, replacing a compressor cost more than buying a new refrigerator. Today, you can replace your compressor for a fraction of the cost of a new refrigerator with this easy-to-install compressor replacement that works with R-134A coolant, considered to be non-hazardous. The 1/3 horsepower is an industry standard that runs off your household 115-volt outlets.