You need to first check and see if the laptop is running hot. You would need to install a program that can monitor it. If it is over heating, that would explain the fan sounds.
You may just need to clean out the laptop of dust and debris to get it all sorted. You may in turn need to replace the thermal paste for the fan(s). Or in the end the fan(s) may need replacing.
NOTE: Before opening up the laptop make sure it isn't still under warranty. If it is, opening it up could void the warranty.