Tube amp blues

I'm sure that there is a simple explanation but being new to tubes I just haven 't learned it yet. When I turn on the amp it starts with no problem and plays wonderfully. After about 3 - 5 minutes it starts making a "static-y" sound from the right channel. This is not constant, it comes and goes about every 10 to 30 seconds and there sometimes is white noise between the louder static-y sounds. Both sounds (Static and white noise) come at different levels sometimes louder than others. Sometimes there is no "static-y" sound at all and all is quiet, just great sounding music. I have done some checking and this sound does not respond to any of the other equiptment. Only the amp needs to be on for it to happen. When the whole system is on there is no difference in the noise. I have played music from my CD and Turntable at different volumes and the noise is not affected. I have found nothing that can be done to stop the sound or make it worse except turn off the amp. It only starts after the amp is warmed up. Could this be a bad tube? Or a bad component on the board that is causing this after the amp warms up? Any Idea's.

Thanks Rick