My first inclination was to pull the driver tubes and shuffle them around to see if the noise moved or even disappeared. For some insane reason I also pulled the rectifier tube at the same time. I then realized that this would complicate matters so plugged it back in.
However in the process of plugging it back in, I somehow did not align it correctly (even though there is a tab on the tube base) and with the pins in the wrong holes turned the unit on.
Rectifier tube did not light, even though the driver board tubes did along with the power tubes. I shut the until off, realized my mistake and re-inserted the GZ34 properly.
Turned on the unit, but the GZ34 didn't light.
Shut the unit down, inserted a fresh GZ34 and fired the unit up.
Everything sounds fine now. Not sure if pulling/re-inserting the driver tubes cleared the crackle of if the original rectifier tube was going bad in the first place.