check your sources and see what maybe causing the noise. If all sources cause the noise then the radio could be the problem.
If you only hear the noise through the amped speaker, I would check the ground, and make sure it is bare. If all else fails you can get a noise suppressor. I actually brought three of them, and I was able to trace my Aux source on my radio as the culprit, and I zapped it with a line noise suppressor.
The trick is to determine exactly what in the audio system is causing the noise.
Crutchfield says the same thing that I am saying. And they have a lot of additional information on diagnosing the problem.
"Are you hearing engine noise?
Any amp installation can be prone to engine noise. You may hear a whining or clicking sound (its pitch will vary with engine speed). Since most noise problems come from a poor ground, always make sure your ground wire is connected firmly to bare metal in the event of a noise problem. You can also try isolating your amp from the chassis of your vehicle by mounting it on a board or using rubber grommets or feet; using a different brand of patch cables; or, only if all else fails, installing a noise suppressor . The tricky part is figuring out which step or steps to take. Please read the rest of this section and try some of the simple tests."
Noise Suppression Guide - Noise Suppression Guide