I just purchased a Marshall AVT20 at guitars center. I like the sound of this amp compared to comparables in the same price range - esp for blues that I play. However, in the quite of my home I noticed the amp has a very audible buzz that I can make louder or quiter by adjusting gain and volume? Is this normal?

It does this even if there is not guitar plugged in, so it's not feedback.

Oh ya, this was the floor model I purchased for about 10% off but and they couldn't find paperwork for it -

Thanks, Brace.