Q: Macbook Pro fan running at high speed but is not experiencing heavy usage

My computer is 3 years old and I've had no problems.

A few days ago the fan started whirring incessantly after the computer had been on for only 10 minutes or so, even when experiencing minimal usage.

This could be just a coincidence but the issue started about the same time I installed the latest security update, I'm on Yosemite.

Can anyone help?