Ground wire?
 

Sometimes my radio gets a hum or a whine sound. I wonder if it is a ground wire?

In a lot of cases the radio is grounded to the chassis by being mounted to the metal frame in the dash, as that is normally the common ground in a vehicle.

But there are a number of possibilities. Bad antenna wire. Bad filtering caps in the radio itself. If the hum changes tune based on engine speed, then something in the ignition circuitry is putting out excessive radio frequency noise and being picked up by the radio. Heck, even a bad speaker can induce it's own hum from a foam separation in the driver or a coil being off center and rubbing against the center post it surrounds.

Actually, sometimes you can eliminate noise by disconnecting one or more audio components from ground, as since everything in the vehicle is connected to common ground, that means injected noise will be there too from any another component connected to that common point.

In my home theater system, I had to get rid of a really bad hum in one of the subwoofers by cutting off the ground wire in the power cord. You have likely noticed that in many instances audio equipment will not have a ground lug on the power plug. That is to isolate that unit from ground to try to prevent any injected hum coming from the unit into ground or picking up existing noise from ground.

Chasing down hums in any audio system can be a pretty time consuming and frustrating experience. But all electronics produce RF noise. That is why they are all normally tested and spec'd for a rated S/N ratio (signal to noise ratio). Some are just going to be better than others, and like anything else, things can fail in them changing that ratio.

No clue what to look for.

I guess you could start here -> https://www.google.com/search?client...m+in+car+radio

Odd it does it off and on.

In a case like that, I would assume a bad connection somewhere that is making and breaking contact. Check the antenna first. Then if there is a dedicated ground wire, check that.