In 10 years of 3100/3200 printing, I've had several printhead failures that resulted in two channels mixing, which were always solved by a fresh printhead. I suppose that it is possible for a printhead to pick up ink from the service station, but IME it is more likely to be the carriage itself. I don't think that there is a vacuum pump in the service station. The odd noises are mostly the pumps pressurizing the ink (supply vs. waste).