in my dads oldie comp ive got a amd athlon xp-m 3000+ in there with a thermaltake volcano 10 on there and this thing on idle is screaming! ive got 7 fans on my own personal computer and all of them combined arent even nearly as loud as this one fan on my dads comp.
dust was probably filling gaps between bearing . once they start getting old, just toss em out. in general, i meant dusting for the heatsink itself.. once i found mine almost full.. never again! im sure with as much canned air as i buy now, they think i am huffing it.
i also agree it will be more work to replace heatsink, so if you can find it, just replace fan.