Noisy tube?

Primaluna evo 100 preamp feeding Primaluna Dialogue Premium (kt150’s) feeding Klipsch Cornwall’s .  About 2weeks ago a significant hum in right channel appears. Very loud.  I start the process of eliminating ground problem.  No luck. Switch cables and hum moves to left side. Simultaneously I develop an intermittent loud static burst on warmup.  Duration of static is about 1-2 seconds.  Very intermittent and amp never goes into protective mode. Replace power tubes with new kt66’s. No change.  So I pull signal tubes one at a time and replace with new tubes.  Right middle slot replaced and the hum is gone.  Replaced the other two signal tubes on the right, just because….I still have this intermittent static crash,, only @a minute after turning on the amp.  Duration of just a second.  Sure sounds like a tube acting up during warmup.  Amp never enters protective mode.  So, any opinions on this?  Appreciate any input from you folks.