Compressor squeaks as pressure begins to build. Not from belt. Does not squeak when no pressure in tank.

Air leaking back from tank check valve or leaking reed valves can make a squeaking or vibrating noise. With pressure in tank remove fitting at the tank check valve and test for leak with soapy water. If leaking, replace with generic valve. If no leak, reinstall fitting and let all the air out of the tank. With empty tank and air filter removed, start compressor and feel strength of suction at intake. When you hear the squeak retest strength of suction at intake and compare. If reed valves are leaking, you will feel less suction as tank pressure increases, and in severe leaking valves, you will actually feel air blowing out of intake. Repair/replace reeds and /or leaking gaskets in head of compressor. Good luck