How old is it? Older laptops tend to clog up their heat-sinks with dust causing them to overheat and make the fan spin faster.
However, if you hear the fan actually grinding the bearings in the fan might also be broken. If this is the case, you need a new fan.
For the dust issue: The best solution is to open it up and clean the dust out yourself.
However this does require some work and is semi risky depending on your technical skill.
https://www.youtube.com/watch?v=AD5jpqVoOu0
If you dont want to do this, you can also try to get some compressed air and and blowing it into the air exhaust to get the dust loose.
* Warning: Do not blow for more than a second in a row, the air will cause the fan to spin too fast and break.
* Warning 2: Do not hold the compressed air can upside down. while spraying.
* Warning 3: This solution is only temporary, you are going to have to open it up and clean it eventually.