Vox AC15C1 Buzzing

My Vox AC15C1 is making a buzzing sound when I hold a chord out and let it resonate. I have a less paul runinng through a green rhino, boss overdrive, boss chromatic tuner, and crybaby. I checked all connections on the pedal board. The buzz happens even when I am playing clean, but ONLY when I let a chord ring out. Does this mean the tubes need replacement? I have had the amp for about 19 months, I play it a lot in a band so I have it cranked to about 12 or 1 o'clock all the time. (Note: I always let it warm up and it usually is on for 3 to 4 hours at a time).