Core 2 Duo vs Pentium 4 heatsink

[j7n]

I have upgraded the processor on my computer from Conroe-L to Conroe E6600 on 965G chipset. The only fan I could install is a used one for a Pentium 4. It has an aluminum core, and 4 pins with PWM. This fan spins between 2000 and 2800 RPM at light load and is very loud, and sensitive to load spikes. It sounds like a ball bearing rattling, or a vacuum cleaner with a bicycle wheel, just as I remember it sounding with the horrible P4.

What puzzles me is that the internally reported temperatures for either core with the new CPU are quite low, between 37 and 53 degrees at 27° ambient. The frequency multiplier switches between 6x and 8x, does not ever reach 9x (not even when it is cool). The Conroe-L single core CPU reached 55-58°C (as reported by itself), but the fan stayed at 900 RPM the whole time. Its original half height heatsink also allowed to reach these temps before revving up.

Could the thermal contact be poor? The CPU never goes above 53 degrees at full load though. I thoroughly wiped the old thermal paste. Is there a fundamental difference between stock Intel LGA 775 fans? What could explain the fan being driven harder with the same or lower temperatures? Is the logic for that inside the CPU? Can the temperatures be read wrong?

I tried SpeedFan software. The computer appears to function fine with the fan at 900 RPM, which unexpectedly corresponds to PWM 60 % (3000 RPM is 100%). I'm not happy to rely on a software because it could fail with the fan stopped.

Fitting an aftermarket HSF on this board is difficult because of the tall northbridge heatsink.