I'm not sure how the Golden Tube is set up, but Najo is right, this sounds like it could be the small signal tubes, rather than the output tubes (the larger ones). Try turning off the unit and swapping the small signal tubes between channels (if there is a way to do this). If the noise switches channels, you have probably found your culprit. If not, turn off the unit again and swap the output tubes between channels, see if that does anything. If after swapping all tubes from one channel to another the noise doesn't move to the other channel, you probably have a problem inside the unit, and should contact the manufacturer. Good luck.