Try some talcum powder, works for months at a time, and makes the engine smell of babies bums!!
No seriously it works, it creates a tiny amount(enough) friction between belt and pulleys to stop the noise.
I did actually mean the timing belt can be noisy in the first reply, but if it is the "fan" belt as it went quiet, then it's that obviously, different makes can be very different in this area.
My dads mondy is done about every 3 months for instant cure, with a tiny amount sprinkled on the inner ribbed section of the belt.
tabetha