Could be one of the ancilleries, such as water pump, P/S pump or alternator. A possible check for this is to remove the drive belts and run the motor briefly. First lose the P/S belt and start up. If all is quiet then suspect P/S pump troubles. If no joy, whip off the alternator/water pump belt and try again (obviously don't run for too long in this condition as there's no water circulating). If taking the alternator and water pump out of the equation doesn't shut things up then you'll have to look else where but at least you'll have knocked three potential culprits off the list.
S.M.