Dimension E520 Fan failure?

I hae a 2 year old Dimension E520. Recently the CPU fan has started coming on at high speed after it has been on for a few minutes. The time before the fan comes on is getting shorter. I've taken the side panel off and it does not feel particularly warm. The operation of the system does not seem to be affected and I am not running any CPU intensive applications.  When I reboot I get a previous fan failure message.