Check to see if it is hooked straight into one of the motherboard headers or if it is hooked directly into the PSU via a molex connector. If it's hooked into the mobo, then that is most likely why. If it is hooked into the PSU then you should probably get a new fan (this happened to me once, it was a crappy fan).
I agree with them it should be a temp sensor you should download an utilitie like SpeedFan to check if it is that. Also check on the BIOS some mobos detail that and give an option,but I think you should let it alone. Good Luck!!
The sensors in fans only measure RPMs, not temperatures. The sensors that read the temperature and control the fans are on the mobo, so you can enable and disable that feature in the BIOS as PR Gamer was saying.