Question 78Refrigeration & AC - QMED

The correct answer is D) foaming of the crankcase oil. When a compressor is heard to knock while pumping down the low side for repairs, but otherwise the compressor sounds normal, it is a possible indication of foaming of the crankcase oil. Foaming of the oil can occur when air is introduced into the system, which can lead to a loss of oil viscosity and cause the compressor to knock. This is because the air bubbles in the oil reduce its lubricating properties, leading to increased friction and wear on the internal components of the compressor. The other options are incorrect because they do not directly relate to the specific symptom described in the question. Worn piston rings or faulty bearings may cause compressor issues, but they would likely result in more persistent or severe problems, rather than just a knocking sound during the pump-down process.