I'm no hardware expert but a few simple pointers:
Firstly, before you open up any computer, make sure that the PC has been unplugged (not just switched off) at the mains and that you are earthed using a wrist strap designed for the job. This will prevent any static electricity from building up and ruining any components.
It's an old laptop, out of warranty so you have nothing to lose by opening it up. Go round each component and make sure everything, especially the fans, are firmly seated. Fans are typically screwed in, but something else might have come loose that's chafing against the fans as they spin.
So make sure that everything near the fans are seated properly. Everything is usually quite tight in a laptop, but just push everything gently but firmly to make sure they are in properly.
It's worth blowing out all the dust at the same time.
Another word of caution though - make sure you put your screws somewhere where they can't roll away and make a note of where everything came from so you can put it back together again afterwards!