Intel Retail HSF Question

DPGX

I recently purchased a 2.66Ghz Celeron D and swapped the heatsink off for an aftermarket heatsink. I was curious as to cooling performance so had swapped back to the retail heatsink again to try it with aftermarket thermal paste. After reseating the heatsink and mounting it, I turned the PC on and noticed the fan no longer fluctuated in performance. Originally it seemed to speed up as the chip's temperature increased, while it now idles at approximately 2500RPM (before it would peak as high as 5000RPM). Im just kind of curious if anyone knows what would cause this?

Thanks in advance,

Mike