Clean the fan out as a first step. If this solves the problem, well and good.
However it is probable that the fan is dying. Replacement should not be either difficult, or expensive, with one warning. Before opening the power supply, uplug the mains lead, and turn the computer on! This will discharge most of the internal capacitors. Even so, take care, as these capacitors can hold enough charge to make your eyes water!
Usually, when the PSU is opened, the fan lead can easily be unplugged from the internal PCB. If it is soldered in, cuttin the leads, and spicing the new fan wires to them should not be difficult. Make certain that nothing gets trapped on reassembly.
be wary of strong drink - it may make you shoot at tax collectors, and miss!