High pitched sound
You hear this from the condenser unit as soon as your unit is on. The sound would usually linger up to 15 seconds. In this case it may also shut off and turn on frequently. The cause is most likely to be high pressure in the system. This is causing the compressor to scream. If you hear such sound shut the A/c off immediately. Running such a unit could be dangerous. Contact HVAC Repair Company near your house to come and take a look at the unit.
Clanking, hitting and knocking
This kind of sound is heard when the blower assembly becomes loose. The fan comes loose from the mount and wobbles and hit against the cage that houses it. There could also be an obstruction in the way of blade resulting in the sound. Turn off your unit. Keeping it on can cause further damages to the unit. Rattling sound is also heard when any of the component of the system comes loose.
Fizzing and gurgling noise
If this is the type of sound you hear from the system, there could be two underlying causes of this- the refrigerant is leaking and the sound is coming from condensate drain. If the leaking refrigerant from the copper tubing is creating this sound, call a technician ASAP. If the sound comes from the condensate drain, make sure that the drain has a trap.