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I thought a ground loop was a bad thing.
Basically it is. This is a difficult subject and books are written about it. It's been a major issue with computer logic and high speed design for decades now. At this level these days we worry about things that I need reading glasses to even see.
The first thing to grasp is simple, just that if electricity is flowing, like water, it is seeking a different potential. So if there is current flowing on a ground wire, it cannot be the same voltage at both ends.
A "ground loop" occurs when you have connections between grounds that are at different potentials. For some reason there are currents flowing on the grounds and when you connect these two grounds, now current will flow thru that connection in addition to how they were flowing before. In a car it's normal to have current on your grounds, they are half the circuit. In a house, it is mostly not normal to have currents on your grounds.
This can be an issue if you are trying to measure things with sensitive instruments, that would include audio equipment (especially turntables), various sensors in general and communications equipment. If you have a record turntable, it measures thousandths of a volt while the stylus follows a groove on a record. If that turntable is plugged into a different branch of your house's system than the amplifier, there is a connection now between the ground shield of it's cable that goes around the house to the chassis ground of the amplifier. If any current runs on this it can put a signal on the wires inside the cable.
For older cars this didn't matter very much. Modern cars are trying to sense things with their ECU's and communicate between various electronic pieces, so they can be sensitive to this.
I know an engineer responsible for an update to a popular and important airplane which entered service in the 70s. More and betterer stuff was required and well into this process he realized the grounding in the plane would not support this. All of us reading this together will not earn enough money in our lifetimes to pay for this cost overrun...