Hi. My amp has been making this really odd sound lately. It's like the sound of wind...when it's really windy. I think I've narrowed it down to somewhere between the gain and EQ in my preamp because 1) It's there when I set the gain to 0, 2) I can EQ it out if I set all the EQ sliders to 0, and 3) I can make it stop by turning the master to 0. It's a Groove Tubes preamp from the early 90's, if that helps at all. Basically the same thing as the SWR 220 preamp, I believe. Any ideas on what this might be and how I might fix it? Has anybody had this problem before?