Sounds like the charge is low, the compressor may also be failing, it should
be almost silent.

   AC compressors can become rather noisy as they become older. While any
noise is not a good thing I have seen compressors run for months even years
with slight buzzing noises. Knocking or grinding noises are a different
story, they should be attended to in short order.
   When your AC stops cooling check to see if the compressor clutch is
engaged. If it is, it may only be a refrigerant issue. If the clutch is not
engaged CAREFULLY unplug the wire from the clutch and probe the harness side
of the connector with a test light.If the light lights, you more than likely
have a bad compressor clutch. If the light does not light the problem is
electrical.
   The over heat switch you are referring to was not used on all models,
however, it is easy enough to locate and test. If you follow the harness
terminal you just checked with the test light, it will either go directly
into the wiring harness or around the back side of the compressor and into a
little round screw-in plug. Another wire will come out of this plug and into
the wiring harness. That screw-in plug is the over heat switch. You can
bypass the switch by  simply rerouting the harness wire over to the wire
coming off the compressor clutch.