I would say it's probably the CPU-fan. Just as Aoi said, clean the fan, BUT remember if you use compressed air, hold you finger in the fan (Yes the computer is turned off while doing this, sorry, just making sure) so the fan can't rotate. The reason for that is to protect the ballbearing from damage.
This could actually also be the case... That the CPU-fan is old and "worn out", so the ballbearing is making a lot of more noise.
Or it could even be the thermal paste, that needs a replace.
But I would go for cleaning it in the first place
Good luck.