when i turn on the ps3 it sounds like a rattle/rumble but then the sound gradually goes away after about 5 minutes. and also when i turn it off it does the same thing. i can play fine and everything but its just that.
whats wrong?
how can i fix this myself (if possible)?
will the sound gradually go after a few weeks of playing?

Could be something to do with the fan. Do you have the receipt, if you do you probably still have a warranty on it, package it back up and send it back. Or take it back to the store you bought it. Good luck.

it isnt the PS3 its ur tv it not compatible properly and also your cabled must be faulty and the rattle is just a lose screw on the fan

faulty.
But the good news is that, if you get on the phone, you can get the ps3 exchanged when the pickup driver comes to collect the faulty machine. Happened to me and got a new one in like 24hrs.

you should check out this site where you can get a playstation 3 for free (hey, that rhymes!)