What actually controls fan speed?

[chmedly]

I just changed the top (the plate where the keyboard, trackpad and power button live) on one of my old A1212 late 2006 17" Macbook Pros and now the fans run 6k just about all the time. The computer was just cleaned out a few weeks ago so the fans are not clogged. The temp sensors are showing very low temps (like less than 105F on all sensors) and the computer feels quite cool (and this machine can get hot when it's doing it's boogie). I have tried the smc reset (disconnect power and hold power button for 5 seconds) and that didn't seem to affect it. When I put the computer to sleep and then wake it back up the fans start out slow (2k) then gradually rise to full speed. I've found that if I lift the lid it will eventually slow down and when I set the lid back down it slowly ramps back up. These transitions take several minutes mind you. The only thing I can think is some kind of airflow obstruction but I'm not finding anything. And still, I'm trying to figure out why it would even matter if none of the temps are calling for it? Any thoughts?