One of the fans is probably on it's way out. For a lot of bearings that are wearing down, the fans will be loud while it's cold, but as soon as the bearings get a little warm the fans will get quiet again as the heat reduces the friction a little bit. As the bearing wear down more, the noise will continue for longer before the bearings "warm up" until friction from worn bearings get hotter and will speed up bearing wear. Eventually, the fan will make a lot of noise and then seize up and die.
You can try to pinpoint where the noise is coming from with something like a tube. Place one end of the tube on your ear, and point the other end of the tube to various locations that might be making the noise. It's best to do this with the computer on while the case is open. Something made of hard plastic works the best like PVC, and about 2-3 inches in diameter. Soft plastic will tend to absorb the high frequency sounds instead of bouncing it inside the tube, making it harder to hear.