fan noise on wife's computer

[tremmor]

Turn it on and whines loud then noise stops. goes into sleep mode and same thing. Its not dirty and has been cleaned. been doing this for a while now. It sounds like its the fan in back of tower. If i take off the side cover i cant tell because it does not make any noise. turn it off and turn it on there is no noise. How can i tell if its the fan on back of tower. I can only suspect it is.
thanks

 

[voyagerfan99]

Unplug the rear fan and set it back where it belongs, then turn it on. If you don't hear the noise, then it's the rear fan.

 

[Darren]

Could be the power supply fan too.