Does the crackling sound like its an electrical crackling? If so try running without the power cable perhaps?
Perhaps something is stuck in the fan. You can try blowing out the dust with one of those compressed air cans? (carefully)
This is really difficult to answer as it could be anything. My guess is that something is loose and you should check it - OR the hard drive some how is damaged?
You will have to go and have it checked by a technician if you can't do it your self.