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?
My Rogue preamp did the same thing.The folks at Rogue suggested cleaning the volume control also.I used a lens cloth,the disposable type glasses cleaning cloth.I folded it out flat,slipped it between the faceplate and volume knob until it touched against the pin that connects to the knob.Then gently scrubbed it back and forth,working all the way around.The cloths dry out quickly so I used three cloths total.No more noise,worked great.