You could have a ground loop, you could have EMI or radiated noise, who knows. If you want to do an install right, eliminating noise is part of the process. Using a ground loop isolator is just a band-aid for the problem, it's not a solution, it just covers it up.
Never let power and signal leads cross or run near each other, if they must cross do so at a 90 degree angle, use ground points with the same electrical potential, use ground points that are as close as possible from the device being grounded, don't use factory wiring since they are often linked to many other electronics in the car, crimp/solder connections properly, etc. All these things are methods for properly installing a system so that you won't get any noise.