The M4A785TD-M EVO apparently ramps the speed of the fan connected to the chassis fan header up and down with the CPU fan speed, even though the chassis fan header is only a 3-pin (non-PWM) header. The PWR fan header does not do this (runs at full speed all the time). Until I figured this out, I actually thought I had a defective fan!
Contrast with the M4A88T-M (a very similar board), where only the CPU fan changes speed with CPU core temp (both the chassis and PWR fans run at full speed all the time).
Nostalgia isn't what it used to be.