Update: the sound came back, so while changing the HDD I followed these instructions:
http://tenonedesign.com/blog/fix-your-macbook-pro-like-a-champ/
Very simple, problem solved, now my nuisance laptop is perfect, solved the spinning beach balls and plinky sound in one swoop. Sweet. Only nervous moment for me was after unscrewing the back plate, it didn't come off easily, i didn't want to pull too hard in case it was fixed in some way I wasn't aware of. Not the case, it was just clipped in, so pull the back off, it's fine 🙂