I have a Mesa 400+ head and the tubes rattle when I put it on top of my cabinet and play. I don't ever recall this happening before. I've had the head for about 6 years. Does anyone know if maybe some tubes could be loose or something? Has anyone evr come across this before? Maybe it's always done that and I just never noticed. It's bad enough that I'm afraid it's ruining something inside the head. Any ideas?