Compressors are not silent when operating. They should run with a monotonous drone. Knocking sounds coming from the compressor are cause for concern. A hum with no background compressor noise may indicate that the compressor is inoperative. No noise at all also indicates that the compressor isn't working.
Scroll-type compressors have a different sound than reciprocating compressors. They have a higher pitched whine. It takes some experience to determine what is typical compressor noise. Some compressors are noisy when new and will break in over time.
Some vibration is common with any compressor. Compressors are mounted on rubber feet to isolate this vibration. Excessive vibration, often accompanied by unusual noise, is a sign of severe problems with the system.
Compressor noise and/or vibration may be the result of -
bad valves, pistons or bearings
a poorly secured compressor (e.g., a broken compressor mount)
slugging (trying to pump liquid Freon or oil)
Unusual noises often indicate imminent failure of the compressor. Excess vibration leads to joint failure in the Freon line connections. This will allow Freon loss which leads to compressor burnout. Internal damage may also be done to the compressor.
Compressor noise can be heard best if you are close to the compressor. Make sure you don't confuse the sound of the outdoor coil fan with compressor noise.
Some people remove the outdoor cover and press the tip of a screwdriver against the shell of the compressor and the base of the screwdriver against the ear drum. This will transmit sounds to your ear without background noise from the fan, allowing a more accurate assessment.
The word Scroll in the data plate will tell you that this is a scroll-type compressor. Make sure you don't misdiagnose the noise that is typical of a scroll compressor as an unusual noise .
Compressors that are slugging may be noisy intermittently. Slugging is the introduction of liquid to the intake side of the compressor. Compressors are not intended to work on liquids. They expect to see a gas. The liquid may be oil or liquid Freon. In either case, slugging is very hard on a compressor.
When the compressor is running, look for evidence of vibration. This can sometimes be seen through the condenser cover or through the coil. Intermittent vibration or vibration which is readily visible may indicate problems. Look at the rubber mounts to make sure they are secure. Don't worry about vibration on startup or shutdown. This is typical. However, a broken mount will show up clearly at startup and shutdown. Where noise levels or vibrations are unusual, recommend further investigation.