Noise when turning the volumen on bottlehead

I have a Bottlehead Foreplay line stage II that recently started to make some noise when I turn the volumen up/down, but the noise is gone when I finish turning the knob. It has a stepped atenuator and in most of the steps make some kind of loud noise in both speakers, the nose can be heard coming out of the speakers at about 12-15 feet. I already changed the two 12X7 tubes and the noise remained. Any ideas about what can I do fix this problem?