Trouble with Vacuum Tube Amplifier

There had been a trouble with my handmaid vacuum tube amplifier that sometime produces a noise in its left channel. The noise ceased by slapping the body:)

I've guessed that the grounding has relevant to this problem but I couldn't specify the undelying cause because usually it didn't occur.
I changed the geometry of the inner circuit. To be more precise, I rearrenged the path of the black wire in the below photo. This wire is the grounding from the power transformer.

The noise in a left channel dramatically disappear so far. Unlike computer programs, building physical equipments is filled with practical know-how which is hard to be addressed without experimentation.