I've handled many T60's, and all types; Intel GPU, X1300, X1400, V5200, V5250, 14", 15", 15.4", Core Duo, Core 2 Duo, XGA, SXGA+, UXGA, WXGA, and WSXGA+.

The OP has done a fine analysis and has documented differences in fan speeds when switching between Core Duo and Core 2 Duo CPUs on the same motherboard.

However, I believe there are more variables in play that can affect fan speed. I believe this because I have seen T60's with the exact same model numbers, BIOS and specifications perform differently. The only apparent difference being the manufacturing date or perhaps the manufacturer of sub-parts within each machine.

My unscientific guess is that there is variance between manufacturers and manufacturing dates for components and/or subcomponents that affect fan speed. This may very well include which LCD panel or fan is being used and may further vary with the specific components or manufacturer of the panel or fan, for instance. Even LCD resolution may be a factor.

I'm just saying I've seen varying behavior even when swapping "like" parts. For lack of a better explanation, it does seem that a quiet T60 can partially be attributed to luck of the draw. I wish it weren't so.

Well, I just turned off the fan on my T5500 1.67GHz GMA950 system and I'm only browsing, no other active processing going on and the CPU went up to 71C I went back to BIOS controlled in TPFC. I suppose I may never be able to get this machine quiet. I confess I'm shopping for a Core Duo T60 as an alternative. The machine also feels fairly warm/hot on my lap (I really do use it primarily as a laptop! I guess I'll tear it open again, reapply Arctic Silver 5 this time, slap a buttered dime on the GPU and hope for better results. Not today, maybe in a week or two. BTW, how do I get a temp reading on the GPU? I'm not seeing it in TPFC.

My idle temps have come down a lot in the few days since replacing the grease on the CPU and north bridge with Arctic Silver 3 (I ordered Arctic Silver 5 yesterday!). I'm idling right now at 34-37C, but the fan is on (a bit under 3000RPM), the lowest possible setting of "1" using TPFC.

How are you getting 37C idling with no noise? Is your fan running? If not, that's some great heat sink installation you have there. I have to think that with your Core 2 Duo, your fan speed can't get below about 2975. That's what this thread is all about. Or you might be running with BIOS fan control, in which case your fan is off a lot of the time. However, when my fan goes off the CPU runs up to over 60C and the fan kicks on momentarily, bringing the CPU temp down to the high 50's and it shuts off again until it goes back to the 60's, kicks in again, and so on...