Sputtering, popping, static, are always a sign of something amiss, and often tubes. Sometimes it can be OK though, like some old input tubes can show a little of this warming up...maybe some bits of loose material creating minor shorts that pass with heating??? But in my experience, with most inputs, power tubes, and with most rectifiers, this is a sign of early failure.

If the sputtering was the rectifier again, regular flashing with noise, from my experience, it will likely go soon.

I did have a Valve Art 274B I used one for some months in my CSP3, and occasionally there would be slight flashing on startup, not regular. I don't think it made noise though. I think I would have pulled it if it made noise. It did not fail, but I ended up changing to other rectifiers, so did not it "play it out" and see if the flashing would lead to worse things.

I don't know how to identify it as an input when there is only one except by eliminating the others. Since you have two power tubes, you can check and see if the racket is only on one channel, then, if it is the power tube, when you change the tube positions to the opposite channels the noise should move too.

It is possible to have bad luck with two bad rectifiers in a row. Might be worth another call to Steve.