Fans do start sounding a bit crappy after time. Might need a good clean of the heatsink.
Get a decent aftermarket air cooler if you haven't already. Will make things a lot quieter - standard heatsinks and fans only do a basic job.
I've had a look and they don't cost much at all, I think I'll just whack one on regardless.
I've not spent a penny in my machine since I bought it 3 years ago, so I suppose it's time it had some tlc.