If the compressor is heard to knock while pumping down the low side for repairs, but otherwise the compressor sounds normal; this is a possible indication of what condition?

Correct Answer: Option D — foaming of the crankcase oil

Pumping down the low side lowers crankcase pressure rapidly, causing refrigerant dissolved in the compressor oil to boil violently. This severe foaming displaces liquid oil and allows liquid slugs to enter the cylinders, producing a distinctive knocking sound.