Loud popping sounds after warm-up

I run a Unison Unico hybrid integrated amp which uses a pair of 12au7 tubes in the preamp side.  Over the weekend it started acting up.  After about 1 - 1.5 hrs of listening (with both tuner and cd), there started a very loud popping sound every 3-5 seconds.  As I was worried about damaging my speakers, I immediately turned things off, so not sure if the sound was from both channels or just one (my wife seemed to think it was only from one channel).  The popping occurs even after turning down the volume to '0'.  Initial thought was that one or both tubes were going bad, but the problem reoccurred after swapping out to another set of tubes.

Would appreciate any thoughts on what the problem might be, or how I could explore the problem further without damaging the speakers.