Could be a bad spark plug wire on the engine or alternator.
Is it a whining noise or popping noise?
With your engine off do you still hear it?
Take a small jumper cable and ground the outer shield on the antenna and see if the noise is still there.
If you take your antenna off and the noise goes away means that you must get rid of the noise at the source (probably engine) because you won't be able to suppress the noise out without hurting your received signals.