ok nino let me lay out the options for you:
Scenario 1- Your PS3 starts up ok but over time gets louder then it's more likely a cooling problem.
Solution 1 - You can try to add aftermarket fans like you suggested. Like I said, if it doesn't help you can return it to your retailer. Just make sure you buy from a retailer with a good return policy.
Solution 2 - If you're in warranty call SONY and see if they're tech support can help you.
Solution 3 - If you are out of warranty and comfortable with dissecting the PS3, then crack it open and clean it up properly. Also, reapply good thermal paste to help keep temperatures down. There are plenty of videos on youtube that shows how to take apart and reassemble the PS3. In fact, I found one that shows me how to fix mine
Scenario 2 - If your PS3 is constantly loud from the second you turn it on to the moment you shut it down, then you may have a bad fan bearing.
Solution 1 - If still in warranty call SONY and explain it to them. See if they will fix it for you. The down side to this is that you have to send in your PS3 and you will lose anything on the hard drive. Back up everything!
Solution 2- If out of warranty, buy replacement fan on ebay and reapply some good thermal paste. Again, refer to youtube for instructions.