You need to get the fan/heatsink cleared of dust/hairs, take it apart (difficult) or blast it through with an air can (pc shop may do this for you)
You're actually recommended to do this every 3-6 months.
Problem is they design them now so you do need to take it apart to get to the fan. You have to be careful with the air can too - very easy to blow the ball bearings out and make it worse.
But yes, in reference to the original comment this is most likely the issue and it's easily remedied by the above solution. Just need a screwdriver and some air can spray, which can be bought for a few quid on Amazon.