Some tubes at start up will have a metal like sound from inside the tube. This is the plate materials being heated up and expand – this sound will disappear once the tube is turned on for a few minutes.

Such a sound will happen at each time you turn on the tube – i.e. at each heat up process. This is perfectly normal and it doesn’t indicate bad tube quality or whatsoever. It’s a natural process of metal expanding under extreme heat.

Some tubes have this sound, others don’t because each type / make of tube might be using different plate materials.