Hi Myles, Newbie here... I have a Rivera Fifty Five Twelve and I get some rattling when I'm on a moderate volume and certain chords/low end frequencies, usually a first position G. I've tried to sit behind the amp while it is happening, but I can't find anything loose or any other sign of the source of the noise. Nothing is coming through the speaker, so on high volume it isn't noticeable, but it's annoying at moderate to low volumes. What should I be looking at? Tubes?