Hi!
The first step I would try is to pull out the left tube and gently scrape the pins with a little pen knife - perhaps there is some oxidation there that interferes. If that does not help, try to move around the connectors to the connecting cables a little.
I doubt that there is something wrong with the headphone driver, but if you switch around the two tubes and you still have the noise in the left channel it may not be the tube but something else.
Good luck!

























