300B 膽有聲, 點算?

I am asking this question for my colleague.

His 300B is using China Tubes. He can for sure hear "noise" coming from the tubes. He turned on the Amp and the noise comes along. It is something like "ng ... ng .... ng" (the sound of electricity)

What could be the cause of this? Please advise.

It may not the problem of the tube, you may need to adjust the bias to minimize the hum sound. Or grounding problem

How to adjust the bias please?