How do I get rid of humming on my amplifier?

I bought a Marshall JVM 210H half stack over a year ago. I recently joined a metal band so I started using heavy distortion. When I played a gig with them, my amp was just feeding back and humming. I started using a noise suppressor pedal which got rid of the feedback but my amp still has the humming sound. Here's the thing, it sounds good when I have it on clean, but when I turn it to the overdrive red mode, it hums. Is this normal? Could it be the tubes?