When I turn the volume past noon on the vib-trem channel, even with no guitar plugged in, a low-midrangey feedback loop will start. If I turn the volume below 9pm, the feedback will go away. The higher I turn up the volume, the quicker the feedback starts. All other channels are fine. I have been googling around trying to find any discussion of this, with no luck. I bought it in 2000, and it has very, very few hours on it. I rarely crank it up, so I have no idea how long this problem has been going on, or if it has always done this. Any ideas?