The drive belt that drives the A/C compressor may be worn or loose . Also the system may be low on refrigerant. If you have a gauge set, check the high/low pressures; with the engine idling and an outside temperature of around 95(F) and Relative humidity of around 50 - 70, the low pressure side should be around 39 - 48 psi, high side around 195 - 245 psi. . If you don't have a gauge set or don't know how to do it, then have an A/C shop do it.