Does anyone know how to stop the hissing noise coming from my car speakers when I comment my iPhone via aux?

That is ampliphonic hiss. Turn the volume down and the phone up, or vice versa, depending on the source of the hiss.

Adjust the volume control on the phone

until you get the best sound quality with the least objectionable noise.

After that, Always use the same setting on the phone

and use the stereo's volume control to adjust the listening level.

If you get a different phone, go through the same procedure again.

We have no idea what "I comment my iPhone via aux" means without more information.

Couple of things: could be the wire picking up interference--try a different wire. Failing that, you can mitigate the noise by turning the stereo down and the iPhone up. This will give you a higher signal-to-noise ratio...it doesn't make the hiss go away, it just makes it weaker than the music signal.