If the device is rattling, it could mean that one of the parts is loose. A nut or screw could be unsecured to the tool, and the vibrations could cause the part to rattle. If this is not the case, the noise could also be attributed to friction. Lubrication of the spindle could suffice. Also take a look here for troubleshooting the angle grinder.
A torn cable can impede the electrical connection to the power source, and it could also be very dangerous. The tear will affect the power flow through the cord, and it could cause extensive heat damage. You should replace the cable. Also take a look here for troubleshooting the angle grinder.
Usually, when the device does not spin at optimal speed, there could be debris and build-up inside the tool. Take apart the head for the grinder and clean out any necessary blockage. This will not only help the efficiency of the angle grinder, but also the life of your tool. Also take a look here for troubleshooting the angle grinder.