To mimick your setup, I placed a ThermoEngine with Delta 38cfm fan on my HSF test assembly. With the CPU running at 70W and the fan at full speed, the temperature of the Athlon XP core, as measured with the internal thermal diode was 41C with the ambient air at 21C. That calculates to .28c/w. Using a rheostat, I started reducing the fan voltage from 12.1V downward. Here's the results:
12.1V 41C .28c/w Loud as you might expect
11.0V 42C .30c/w Considerably less shriek
10.0V 43C .32c/w No shriek
9.0V 44C .33c/w About like a YS-Tech 27cfm
8.0V 45C .35c/w As loud as the Water cooled rig I'm typing on.
I suspect that your Tbird is running more wattage and the inside of your case is hotter than 21C, so your temp will probably be higher, but the relationship of temp to the fan voltage will not change. Good Luck
Hoot