Hi
Most likely the power supply is screaming due to an overload or it can't support the load of the computer. If you are not comfortable working on computers it would be best to have a technician check it for you. In the mean time leave it tuned off at the wall.
If you are comfortable replacing computer parts, plug the computer in and see if you can isolate where the noise is coming from. If it is the power supply please post the model of Packard Bell or some pictures of the current power supply and motherboard so a suitable replacement can be found.