GE microwave -crackles and sparks as if metal inside

The problem you are having is that the magnetron that does the heating is shorting out and it needs to be replaced before it damages the inside covering of the microwave. www.mwoparts.com is a good source for magnetrons or other similar sites. To replace is easy enough. Taking the cover off with the power off, you will find it behind and on the top right side of the control panel. It is held in place normally with just 4 screws and two wires with terminals attached to it. So removing and replacing is pretty straightforward. Just be sure all the power is off until you have everything back in place. Good luck.