Jolida 502B

Need some advice please.
Left channel has "busss" noise even the volume knob is at zero position. Input selector get noise when it is switched from time to time. Anyone has the same problem ?? I suspect dirty knob contact ?? Solution ???
I have a Jolida Sj 502a and it has the same ploblem. I change the out put tubes without success. I thing is bad quality of some parts of amp.

If you would find a solution, contact me.

I also have the same noise in the left channel of my 502B. I usually reduce this effect by re-biasing the amplifier when the noise gets too loud (or noticeable from the listening position). After re-biasing, the noise still exists but it can only be heard by approaching the ear to the loudspeaker. On the other hand, the right channel is absolutely quiet. I live in Spain and neither the provider nor the importer's technical service were able to give me a better solution.