The USB cable is probably creating a ground loop when you connect it. Here are a couple of things to try:
1. Put a ferrite bead around the USB cable.
2. Power the printer and the PC from the same mains power outlet, if you are not already doing so.
1. Put a ferrite bead around the USB cable.
2. Power the printer and the PC from the same mains power outlet, if you are not already doing so.