[Just noticed I wrote 200hz in the title, instead 120. Gah! Wish I could edit that.] My house was built in the 50s. I have just one outlet in the garage. It is powered by an ungrounded cable, and has a GFCI outlet installed. I want to play in there, but my amp makes a loud 120hz buzz when I roll my guitar volume up. There's no buzz with the amp volume up and guitar plugged in. But as soon as the guitar volume is turned, up the buzz comes in with it. I've tried two different tube amps with the same result. There are ceiling lights in the house on the same circuit - several on dimmer switches - but the buzz persists even when they are turned off. I also have a garage door opener, and a microwave oven on the same circuit. Neither of these are running, obviously. Thoughts?