Howling cooling fan.
Left fan on a 1,1 15" MBP was making a awful racket, worn out bearing, had to go. Couldn't stand to use the thing it was making such a fuss.
A cinch - whole job took about 20 minutes start to finish. In a MBP 15" the left fan is pretty easy to get to and get out of the computer. When the new one from iFixit arrived, I put it right in and reassembled - no more racket!
Carefully look over the instructions on how to disconnect the several cables that have to be moved out of the way to replace the fan - especially the copper-colored ribbon cable that lays across the main body of the fan; it attaches to the MLB via an ultra-miniature, verrry delicate 'clamp' style connector. If you try to unfasten it the wrong way you will break it. That would be... very bad.