I own a marshall 1959 HW which i rarely use. Last time i used it was 6 months ago. The thing is that after leaving it on idle and then taking the standby off there is noise when in raise the presence knob to 10 or if i raise the volume 1 knob to 3 or more. It sound like a blowing wind that tends to produce a high pitch sound. What could be causing this and how do i fix it?