A ground loop occurs when there is more than one ground connection path between two pieces of equipment. The duplicate ground paths form the equivalent of a loop antenna which very efficiently picks up interference currents. Lead resistance transforms these currents into voltage fluctuations. As a consequence of ground loop induced voltages, the ground reference in the system is no longer a stable potential.
Yes it's bad.
Try individual ground wires for each coil and ground them each to a different location if you want to go crazy. Or just create a single point ground like the factory system...