Based on the comments so far, the problem is in the preamp (which is to say the cables and amp ar off the hook), but its unclear whether its a problem with a functioning source connected, as Al is asking about.
Since its affected by volume, its ahead of the volume control. The thing is, without an input, this could be perfectly normal. So connect something to the input and see if it still does it.
Since its affected by volume, its ahead of the volume control. The thing is, without an input, this could be perfectly normal. So connect something to the input and see if it still does it.