Help Diagnose Tube Preamp Problem

I have a George Wright preamp along with his mono 3.5 amps. I recently started hearing some noise or crackling through my speakers at various times. When I turned the system on the other day, it was very loud for a couple of seconds, but then went away. I've done some research and "tested" the tubes by gently tapping them with a pencil, and the weird thing is the tubes in the preamp all give off a ping that is not solid like the tubes in the amps (I don't believe the tubes in the preamp all went bad at one time). Even stranger, if I tap on the metal on/off switch, this also pings through the speaker as if it is somehow getting picked up by the current. Something strange going on, anyone with any ideas of the problem? thanks