the left fan on my MBP was making ticking noise so it took it in and they replaced both fans. All was fine for 24 hours but now the left fan keeps kicking in at full speed for no reason - the machine is cool and activity monitor shows nothing much is going on. It even does it straight after a re-boot I've tried to zap PRAM COMMAND+ALT+P+R at reboot but it fails to work - no second chime.... any ideas ?????