While it is not a fault, if the noise offends you, it may be that the friction surfaces have become glazed.
Dismantle the unit and, if the friction surfaces are shiny, take the shine off with some emery paper. Make sure that any residual emery is removed. Reassemble and it should be quiet. As "parklands" says, make sure that you reassemble in the reverse order of dismantling so that the discs and friction discs are in the correct place.