A knock in the compressor may be caused by a loose connecting rod, eccentric strip and rod, eccentric disk, piston pin, crankshaft, or too much oil in the system. A compressor knock can be determined by placing the point of a screwdriver against the crankcase and your ear against the handle. A knock can then very easily be heard. It will not be possible to determine what causes the knock before the compressor is disassembled. Sometimes it may be possible to determine a looseness of the aforementioned parts without completely disassembling the compressor.
First, remove the cylinder head and valve plate to expose the head of the piston. Now, start the motor and press down on the top of the piston with the finger. Any looseness can be felt at each stroke of the piston. The loose part should be replaced. It is always good to check the compressor oil level first before analyzing and determining the compressor repairs. Oil knocks are usually caused by adding too much oil in servicing.
It should never be necessary to add oil to a system unless there has been a leakage of oil. A low charge is sometimes diagnosed as lack of oil. Always ensure that a low oil level is actually caused by lack of oil rather than a low charge before adding oil to the system.