they may have replaced something just to replace something.
The only sensor in the fan is a tach sensor, to let the computer verify the fan is spinning at the speed the computer wants it to spin at. If the fan is gummed up, the computer can send the fan more power if needed, to meet the target RPMs. If the sensor isn't working properly, the fan is usually pushed to max voltage as a safety.
Some macs will push ALL fans to max if a single sensor isn't working or is unplugged.
Maybe your mac had a bad fan that wasn't able to spin fast enough, and your mac was overheating? I use TemperatureMonitor here for a lot of things. The macbook only hast the one fan so you won't get the sound of other rushing fans if that one has problems. Still, it'd be nice if the mac would pop up a warning or something? Though in all the cases I've seen, an overheating macbook just suddenly shuts off.
