Why my car stereo is making a humming noise ?

Everyone so far had answered somewhat right but there are many other things that can come into factor. Ground Problems--- Check to make sure the paint is removed from the area that your ground is attached to. Also make sure your ground is no longer then two feet. "Longer the ground better chance you will have problems". Make sure the ground is attached well."not loose" and attached to solid metal " Not a real thin part of the body" Power Problems-- Make sure your power wire is ran on the other side of the car from your RCA wires. Make sure the wire has a good connection to the battery and amp. Make sure the remote wire is well attached to the radio and amp. Radio Problems--- Make sure the radio has a good ground connection."The connection between the factory wires and the radio." These can all cause the same problem. I would start with the first thing I said and work your way through the list.

Yes, the grounding is a so important issue in this matter. also check your "shield" of sound wirings. I mean the wires related to speakers(input & output of Amp.) should covered by shield.

you ain't grounded right! remember always remove your negative terminal on the battery before jacking around or you could french frie your hair extensions.

it's not hooked up right. do you have a good ground? did you do everything the correct way?

I'm not sure. Maybe get a mechanic to check it out. I might need to be changed.