+1. Ground issues seem to be the most common. Wraitherlx has a pretty good background in electronics, as does Safi, and many other members. They should be able to help you figure out the issues rather quickly.
with a factory system this problem is normally caused by the alternator. The alternator has many diodes or rectifiers in it. Alternators are designed so that if one or two diodes fail, the alternator still works just fine. Diode failure shows up a radio noise slowly and for apparently for no reason. These diodes as they fail cause ripples in the electrical current, it is no longer smooth, and the radio picks these ripples up, you can hear them. There are also sound filters, commonly called capacitors, caps, or condensers. They are just different names for the same device. Any way it is common for your alternator to have one of these sound suppression devices. It can be internal or external to the alternator. If one of these fail you can get the sound you are speaking of. The sound is not normally caused by a ground unless you have a custom radio install and the ground wire is excessively long, or grounded to the wrong place.
If you purchased your car used you may want to check behind the radio to make sure there arent any exposed wires or extra wires from a system that may have once been in there but im shooting for the above.
Also, on the back of the block there are Radio Noise Capacitors on each side of the block. They are located between the back and the firewall. They eliminate radio frequencies that may be outputted by a component to ensure that it doesnt interfere with another component. Make sure that those wires are still connected and intact as Ive worked on numerous new edge mustangs that the problem was attributed to them. another thing to check too is to make sure that your battery terminals are clean and tight.
how old is the battery? do you have access or know how to use a multimeter? I would take you car to an autozone and have them check out the battery as well as the alternator to make sure that they are up to par.