You may be seeing an old fan wearing out. At least such are widely available where I am.
My Prescott fan used to get around 4000 RPM I think, made heaps of noize when the cpu was at max.
But Nowadays it wont get above 2428 RPM.
I did some research and tried disabling thermal control, Q-fan mode (or what ever) & adjusting the ratio up n down, also disabling q-mode, but no luck...
When the Prescott's get to hot they throttle themselves down yes??? Thus dramatically reducing my performance especially considering I get approx 75 degrees C, under 25% load.
Any input much appreciated!!!