It's telling you you should be playing longer.
I haven't got a PS3, but when you 'turn it off' isn't it actually going into standby... if the noise you are hearing is the fan it may just be having trouble cooling down (maybe an emergency feature allows it to turn it's fan on even on standby)
Leave it for a while and it will hopefully cool down. (maybe if it is struggling to cool down it is full of dust, which i hear is quite common for PS3s, I believe the usual advice is to try vacuuming the air vents or spraying a can of compressed air into it (best turn it off proper for this though, unplug it)
If it's a disc reading noise then I dunno, just cross your fingers and hope it doesn't do the YLoD (which if I recall is a problem when turning on the PS3, so I guess it's probably not that)
