Loud banging and squeaking

that depends - it sounds like fan motor going out or a foreign object. With a flashlight go outside and observe the unit running ( ideally during the malfunction ), you will want to shine the flashlight into the louvers of the case so that you can see inside the unit (( depending on construction)). Look for bent / loose fan blades and/or foreign materials. If no obvious obstructions are found you will need to consult a qualified person. To give you an idea of what you can expect I will construct a repair scenario :
Service call : $65 - $90
Fan Motor : $ 80 - $150
Fan Blades : $ 30 ( hopefully you can re-use your old one if it is not damaged)

The only other thing that it could be is the compressor, if that is the case don't bother repairing. Get a new unit.
Take my example, add it up and compare it to a new unit with a least as much or more BTU output. If repair costs are 50% or more of a replacement unit I would recommend replacing it with a new one.

A few words on selecting the right repairman.

If you choose to consult a repair service check to see if they are listed with the BBB.
Ask around - friends & relatives may be able to point you to somebody.
Take advantage of vocational programs in your area : Most vocational schools have heat and air classes. The young people attending these classes will sometimes repair your unit for parts cost to get the practice ( check with the school)


Hope this helps