Probably a play in one of the cooling fan axles.
Mine makes a sound like that from times to times, especially after being on for a long time, or when there is alot of processing. (graphics card or processor gets hot, and the fan goes to its faster speed).
I just give it (the PS3) a gentle, but firm punch, sometimes you need to punch it several times before it goes quiet.
This however does not fix the problem, it will eventually come back, and over time get worse and worse.
That said, its over 3 years since mine started doing this, and its still not that big of an issue.
First thing you should do though, is grabbing your vaccum cleaner and vaccum out all the accumulated dust in the console, dust is the enemy of the cooling system, and if you're unlucky, it can short-circuit several components in the system aswell. Get it out and it should suffice with the lower speed on the cooling fan, hence reducing the risk of it starting to make that sound.
If you have the knowledge, open it up and dust it off with a soft brush (dont use the vaccum cleaner inside the console, the EMP from it can burn the motherboard circuits if you are not carefull).