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.
