Don't forget Trixta, the temperature in the UK has gone from 15 degrees to nearly 30 degrees in about a week! Your fan is probably just having to work harder.
1. Summers here! Which means it takes a more fan-power to cool off your computer.
2. You recently added something inside the machine which makes it hotter which can result in the extra fanning.
3. Your fan might be dying or malfunctioning. If you have an extra power supply, try to switch it out and see if theres a difference.
If anything, you can try to install some extra fans into your chassis or in a slot to throw out some hot air and put in some cool air and see if that helps. Hope my rookie advice helps out!