Medium pitched sound from speakers

Last night when I turned the system off it was working perfectly. Today when I first turned it on there was a low to medium pitched tone coming from the speakers, even with the preamp volume turned all the way down. When I turned the preamp back on the tone returned. I checked for loose connections but couldn’t find any.

What could be causing this? Could it be a tube in the preamp going bad. The amp is solid state.